SOUTHERN  BRANCH, 

UNIVERSITY  OF  CALIFORNIA, 

LIBRARY, 

JLOS  ANGELES,  CAUF. 


FIRST   PRINCIPLES 


NATURAL  PHILOSOPHY. 


A    TEXT-BOOK 


FOR      COMMON      SCHOOLS 


ELROY    M.    AVERY,    PH.D., 

AUTHOR    OF    A    SERIES    OF    TEXT-BOOKS    ON    PHYSICAL    SCIENCE. 


NEW    YORK  •.•  CINCINNATI  -.-  CHICAGO 

AMERICAN    BOOK    COMPANY 

67361 


DR.    AVERY'S 
PHYSICAL    SCIENCE    SERIES. 


I  St. 

FIRST    PRINCIPLES    OF    NATURAL     PHILOSOPHY 


THE    ELEMENTS    OF    NATURAL    PHILOSOPHY. 

3d- 

THE    ELEMENTS    OF    CHEMISTRY. 

4th. 

THE    COMPLETE    CHEMISTRY. 

Containing  the  ELEMENTS  OF  CHEMISTRY,  with  an  additional  chapter  or 
Hydrocarbons  in  Series  or  ORGANIC  CHEMISTRY.  It  can  be  used  in  the  sair« 
cl«ss  with  THE  ELEMENTS  OF  CHEMISTRY.. 


CajyH&f,   1884,'"$  'Sheldon  &•  Co. 


A. 


THIS  book  is  the  result  of  an  attempt  to  meet  the 
wants  of  schools  which  cannot  give  the  time 
required  for  the  completion  of  the  author's  Elements  of 
Natural  Philosophy.  No  effort  has  been  spared  by 
author  or  publishers'  to  make  it  worthy  of  the  place  it 
is  intended  to  fill. 

Especial  care  has  been  taken  to  provide  simple,  teach- 
ing experiments  which  do  not  require  expensive  ap- 
paratus. 

The  latest  developments  of  science,  such  as  the  intro- 
duction and  use  of  electrical  units,  have  been  freely 
utilized  ;  the  mere  polemics  of  physics  has  been  ignored. 

Any  teacher  or  pupil  using  this  book  and  finding  any 
error  therein  is  requested  to  communicate  the  same  to 
the  publishers  or  author. 

If,  in  its  study,  any  person  encounters  difficulties  not 
easily  removable  by  other  means,  he  may  feel  free  to 
write  to  the  author  (in  care  of  the  publishers)  for  fur- 
ther information.     Such  letters  should  be  accompanied   \ 
with  addressed  envelopes  for  replies. 


CHAPTER 


MATTER. 

SBC.  PACK 

I.  MASSES,  MOLECULES  AND  ATOMS.., 1 

II.  PHYSICAL  PROPERTIES  OF  MATTER 10 

III.  THREE  CONDITIONS  OF  MATTER...  .     20 


CHAPTER  II. 

MOTION  AND  FORCE— DYNAMICS— GRAVITATION 
—ENERGY. 

I.  MOTION  AND  FORCE 26 

II.  GRAVITATION  36 

III.  FALLING  BODIES 45 

IV.  PENDULUM 51 

V.  ENERGY...  ....  67 


CHAPTER  III. 

SIMPLE  MACHINES. 

I.  PRINCIPLES  OF  MACHINERY— LEVER 67 

II.  WHEEL  AND  AXLE— PULLEY 78 

III.  INCLINED  PLANE— WEDGE— SCREW 80 


vi  CONTENTS. 

CHAPTER   IV. 

LIQUIDS. 

SBC  PAGE 

I.  LIQUID  PRESSURE 96 

II.  EQUILIBRIUM— BUOYANCY 107 

III.  SPECIFIC  GRAVITY 113 

IV.  HYDROKINETICS 117 

CHAPTER  V. 

PNEUMATICS. 

I.  ATMOSPHERE— ATMOSPHERIC  PRESSURE 122 

II.  PUMPS— SIPHON 129 

CHAPTER  VI. 
ELECTRICITY  AND  MAGNETISM. 

I.  GENERAL  VIEW  140 

I!.  KRICTIONAL  ELECTRICITY 151 

III.  VOLTAIC  AND  THERMO-ELECTRICITY 180 

•V.   MAGNETISM 207 

V.  INDUCED  ELECTRICITY 227 

CHAPTER  VII. 

SOUND. 

I.  NATURE,  REFRACTION  AND  REFLECTION  OF  SOUND...  245 
,'l.  TELEPHONE— COMPOSITION  AND  ANALYSIS  OF  SOUNDS  259 

CHAPTER  VIII. 

HEAT. 

I.  TEMPERATURE— THERMOMETERS— EXPANSION 281 

II.  LIQUEFACTION  AND  VAPORIZATION ..  289 


CONTENTS.  Yii 

SEC.  PACK 

III.  LATENT  AND   SPECIFIC  HEAT 397 

IV.  MODES  OF  DIFFUSING    HEAT 308 

V.  THERMODYNAMICS 817 

CHAPTER  IX. 

LIGHT. 

I.  NATURE,  VELOCITY  AND  INTENSITY  OF  LIGHT 328 

II.  REFLECTION  OF  LIGHT 336 

III.  REFRACTION  OF  LIGHT 347 

IV.  CHROMATICS  AND  SPECTRA 365 

V.  OPTICAL  INSTRUMENTS 373 

CONCLUSION— ENERGY 382 

APPENDIX 887 

INDEX...  ..  393 


CHAPTER    !* 

MATTER. 

SECTION     I . 
MASSES,    MOLECULES  AND  ATOMS. 

"  Read  Nature  in  the  language  of  experiment." 

Experiment  I. — Place  a  cork  on  the  surface  of  water.  Invert 
a  glass  tumbler  or  jar  over  the  cork  and  push  the  glass  down 
into  the  water.  Notice  that  the  water  cannot  enter  far  into 
the  vessel,  although  it  enters  far  enough  to  show  that  its  tendency 
is  thus  to  enter,  but  that  something  prevents  it.  The  air  in  the 
glass  prevents  the  water  from  entering.  Air  and  water  cannot 
be  in  the  same  place  at  the  same  time. 

Experiment  2.— Try  to  get  your  two  hands  in  the  same  place 
at  the  same  time.  Repeat  the  attempt  with  two  books  and  other 
articles,  until  you  are  sure  that  no  two  things  that  you  can  handle 
can  be  in  the  same  place  at  the  same  time  and  that  every  one  of 
them  takes  up  room. 

I.  What  is  Matter  ? — Matter  is  anything  that 
occupies  space  or  "takes  up  room." 

Mind,  truth  and  hope  do  not  take  up  room  and  so  we 
know  that  they  are  not  forms  of  matter.  The  earth,  the 
air,  a  table,  an  orange,  a  slate  or  a  raindrop  does  take  up 
room  and  so  we  know  that  each  is  a  kind  or  form  of 
matter. 


%  NATURAL  PHILOSOPHY.  §  2 

2.  Divisions    of    Matter.  —  Matter    exists    in 
atoms,  molecules  and  masses. 

It  is  very  important  that  we  clearly  understand  what 
these  words  mean  or  we  shall  have  trouble  in  trying  to 
understand  much  that  is  to  follow. 

3.  What  is  an  Atom?— An  atom  is  the  small- 
est quantity  of  matter  that  can  enter  into  com- 
bination and  thus  form  molecules  and  masses. 

In  nearly  every  case  an  atom  is  a  part  of  a  molecule. 

(a.)  We  may  say  that  atoms  are  the  smallest  particles  of  matter 
that  can  exist.  They  seldom  exist  alone,  but  quickly  unite  with 
others  like  themselves  to  form  elementary  molecules,  or  with  others 
unlike  themselves  to  form  compound  molecules.  For  example,  one 
atom  of  oxygen  combines  with  another  like  itself  to  form  an  ele- 
mentary molecule  of  oxygen,  while  one  atom  of  oxygen  combines 
with  two  of  hydrogen  to  form  a  compound  molecule  of  water. 
There  are  sixty-six  kinds  of  atoms  now  known. 

4.  What    is    a    Molecule  ? — A  molecule    is   a 
quantity  of  matter  so  small   that  it  cannot  be 
divided  without  changing  its  nature. 

The  word  molecule  means  "a  little  mass,"  but  in 
Natural  Philosophy  we  must  be  very  careful  to  use  the 
word  with  accuracy — that  is,  in  accordance  with  the 
above  definition. 

(a.)  A  molecule  is  so  very  small  that  a  total  of  8,000,000,000 
molecules  of  water  is  barely  visible  in  the  best  of  modern  micro- 
scoi>es.  ll  a  drop  of  water  could  be  magnified  until  it  appeared  to 
Vie  as  large  as  the  earth  on  which  we  live,  each  molecule  in  the  drop 
thus  magnified  would  still  look  sma'ler  than  a  base-ball.  But 
while  the  pupil  may  thus  get  the  idea  that  a  molecu.e  is  very 
small,  he  most  not  think  that,  like  the  fairies,  it  exists  only  in 


§  5  MASSES.  3 

fancy.  Molecules  are  as  real  as  base-balls  and  much  more  numer- 
ous. If  a  molecule,  by  any  means,  be  divided,  its  nature  will  be 
changed.  A  molecule  of  water  may  be  changed  into  two  atoms  of 
hydrogen  and  one  of  oxygen. 

(6.)  In  an  elementary  molecule  the  atoms  are  alike  ;  in  a  com- 
pound molecule  the  atoms  are  of  two  or  more  kinds.  Some  com- 
pound molecules  are  very  complex.  The  common  sugar  molecule 
contains  forty-five  atoms  of  three  kinds.  Elementary  molecules 
make  elementary  masses  or  substances.  Compound  molecules 
make  compound  masses  or  substances.  The  nature  of  the  mole- 
cule determines  the  nature  of  the  substance. 

5.  What  is  a  Mass  ?  —  A  mass  is  any  quan- 
tity of  matter  that  is  composed  of  jnolecules. 

If  a  quantity  of  matter  is  large  enough  to  be  seen,  even 
with  a  powerful  microscope,  you  may  know  that  it  is  a 
mass.  If  it  may  be  divided  without  changing  the  nature 
of  the  substance,  you  may  be  sure  that  it  is  a  mass. 

Masses  are  elementary  or  compound.  An  elementary 
mass  is  called  an  element.  There  are  as  many  elements 
as  there  are  kinds  of  atoms.  Compound  masses  or  sub- 
stances are  innumerable. 

(a.)  We  may  take  a  lump  of  salt,  which  is  a  mass,  and  break 
it  into  many  pieces  ;  each  piece  will  be  a  mass.  We  may  take  one 
of  these  pieces  and  crush  it  to  finest  powder ;  each  grain  will  still 
be  a  mass.  We  may  imagine  one  of  these  grains  of  powdered  salt 
to  be  divided  into  so  many  parts  that  any  further  division  will 
change  them  from  salt  to  something  else.  These  particles  of  salt, 
which  are  so  small  that  further  division  would  change  their  nature, 
are  too  small  to  be  called  masses  ;  they  are  molecules.  If  one  of 
these  molecules  be  divided,  it  will  cease  to  be  salt.  Instead  of 
salt,  we  shall  have  an  atom  of  chlorine  gas  and  an  atom  of  the 
metal  sodium.  Two  molecules  would  make  a  mass,  but  the  mass 
would  be  so  small  as  to  be  invisible,  except  in  imagination. 


4  XATtrRAL  PBILOSOP&Y.  §  6 

Experiment  3.— Heat  the  mercury  in  the  bulb  of  a  common 
thermometer.  The  bulb  remains  full,  but  the  liquid  rises  in  the 
tube.  There  seems  to  be  more  mercury  than  there  was  before 
How  can  this  be  ?  There  must  be  a  greater  number  of  molecules, 
the  molecules  must  be  larger  or  they  must  be  farther  apart. 

Experiment  4. — Heat  some  water  in  a  glass  tube  without  boiling 
it.  Bubbles  arise  and  attach  themselves  to  the  sides  of  the  tube. 
These  are  bubbles  of  air.  This  air  came  from  the  water.  The 
air  particles  were  either  in  the  same  place  with  the  water  mole- 
cules, at  the  same  time,  or  were  in  otherwise  vacant  spaces  between 
the  water  molecules.  Can  you  believe  that  they  were  in  the 
same  place  at  the  same  time  ? 

Experiment  5. — Carefully  pour  alcohol  into  a  test  tube  about 
three  quarters  full  of  water  until  the  tube  is  full.  Close  the  mouth 
of  the  tube  with  thumb  or  finger  and  shake  the  liquids  until  they 
are  mixed.  Notice  that  the  tube  is  not  full  now.  Some  of  the 
molecules  have  been  destroyed  or  dropped  into  the  spaces  between 
other  molecules.  Can  you  believe  that  any  of  the  molecules 
were  destroyed  ? 

Experiment  6. — Make  a  common  goose  quill  pop-gun.  Notice 
that  when  you  use  it  the  air  confined  between  the  two  wads  is 
compressed  or  made  to  occupy  about  half  its  original  space.  The 
air  particles  were  reduced  in  size  or  in  number,  or  were  crowded 
together  more  closely.  Perhaps  the  matter  of  which  a  body 
is  made  does  not  actually  fill  all  the  space  which  the  body 
seems  to  occupy. 

6.  Continuity  of  Matter. —  When  you  look  at  a 
brick  wall  from  a  considerable  distance,  it  has  an  appar- 
ent uniformity  of  structure;  you  cannot  see  that  it  is 
made  of  many  bricks,  separated  by  mortar-filled  spaces. 
This  is  the  fault  of  your  sense  of  vision  and  not  the 
fault  of  the  wall.  As  yon  come  nearer,  you  see  whaf 


§  7  FORMS  OF  ATTRACTION.  5 

you  did  not  see  before — the  individual  and  separated 
bricks.  But  such  is  the  structure  of  the  wall  whether 
you  can  see  it  or  not. 

Rub  the  smooth  handle  of  a  fine  awl  over  a  piece  of 
fine  wire  gauze  and  the  gauze  seems  to  present  a  con- 
tinuous surface.  It  is  the  fault  of  the  instrument  in 
your  hand  and  not  the  fault  of  the  gauze.  Rub  the 
point  of  the  awl  over  the  gauze  and  you  soon  find  open- 
ings between  the  metal  threads.  But  the  openings  are 
there  whether  you  can  feel  them  or  not. 

Rub  the  point  of  a  fine  sewing  needle  over  the  surface 
of  a  window  pane.  The  glass  seems  to  be  continuous  in 
its  structure  and  the  needle  cannot  get  through.  It  ia 
the  fault  of  the  instrument  you  are  using.  Try  one 
more  delicate.  Let  a  ray  of  light  fall  upon  the  glass 
and  it  easily  finds  a  passage  way  between  the  solid  mole- 
cules. Rays  of  light  are  often  used  by  scientific  men  as 
instruments  for  their  work. 

There  are  spaces  between  the  molecules  of  every 
form  of  matter,  whether  you  can  detect  them  by 
any  of  your  physical  senses  or  not. 

7.  Forms  of  Attraction. — Each  of  these  three  divi- 
sions of  matter  has  its  form  of  attraction. 

The  attraction  of  masses  is  called  gravitation. 

The  attraction  of  molecules  is  called  cohesion 
or  adhesion. 

The  attraction  of  atoms  is  called  chemical 
affinity. 


6  NATURAL  PHILOSOPHY.  §  8 

8.  Forms  of  Motion. — It  is  probable  that  each  of 
these  three  divisions  of  matter  has  its  own  form  or  mode 
of  motion. 

The  motion  of  a  mass  is  often  called  mechan- 
ical motion.  The  motion  of  a  hammer  or  of  a  flying 
bullet  is  an  example. 

The  motion  of  the  molecules  in  a  mass  consti- 
tutes heat  or  light  or,  probably,  electricity  or 
magnetism.  If  a  flying  bullet  strike  the  target,  we  may 
imagine  that  the  shock  which  destroys  its  mechanical  mo- 
tion, produces  or  increases  the  vibration  of  the  molecules 
among  themselves.  We  ought  to  have  little  difficulty  in 
imagining  these  little  molecules  rapidly  swinging  to  and 
fro  within  the  mass.  These  molecular  vibrations  consti- 
tute heat.  We  know  that  when  a  bullet  is  stopped  by 
the  target,  the  bullet  is  heated  and  the  production  of 
heat  is  to  be  explained  only  in  this  way. 

The  motion  of  atoms  within  the  molecule  is 
probable,  but  has  not  yet  been  proved. 

(a.)  Fancy  a  million  flies  surrounded  by  an  imaginary  shell.  If 
each  fly  be  allowed  to  represent  a  molecule,  the  contents  of  the  shell 
will  represent  a  mass,  because  it  is  composed  of  many  molecules. 
We  may  imagine  this  shell  to  be  thrown  through  the  air.  The 
motion  of  the  shell  would  represent  mass  or  mechanical  motion. 
As  the  shell  is  moving  through  the  air,  the  flies  are  moving  slowly 
among  themselves  within  the  shell.  This  motion  of  the  flies 
represents  molecular  motion  and  is  a  very  different  thing  from  the 
motion  of  the  shell.  When  the  shell  strikes  the  ground,  the 
mechanical  motion  is  destroyed  but  the  molecular  motion  is 
increased,  for  the  flies  are  set  in  much  more  rapid  motion  by  the 
shock.  This  is  just  about  what  happens  when  the  bullet  is  fired 
against  the  target. 


§  10  CONSTITUTION   OF  MATTER.  7 

(6.)  These  motions  of  atoms,  molecules  and  masses  give  to  mattet 
the  power  of  doing  work,  the  scientific  name  of  which  power  is 

Energy. 

(c.)  Try  to  get  a  clear  mental  picture  of  theso  molecules,  each 
separated  from  its  neighbor  by  a  distance  much  greater  than  its 
own  diameter.  If  a  mass  could  be  magnified  until  its  molecules 
were  as  large  as  worlds,  the  spaces  between  the  molecules  would 
be  as  great  as  the  spaces  between  the  planets.  Try  to  imagine  a 
creature  small  enough  to  live  on  a  molecule  as  we  live  on  the 
earth.  Imagine  this  creature  looking  at  the  neighboring  molecules 
as  we  look  upon  the  stars  at  nigrht.  Remember  that  each  molecule 
is  in  motion  back  and  forth  among  its  neighbors,  sometimes  strik- 
ing them,  perhaps,  and  rebounding  from  them.  When  we  heat  the 
mass,  we  make  each  molecule  move  faster,  strike  harder  blows 
and  push  its  neighbors  further  away,  thus  producing  expansion  of 
the  mass. 

9.  The  Constitution  of  Matter. — Every  body  of 
matter  is  made  of  a  vast  number  of  minute  par- 
ticles called   molecules,  no   two  of  which  are   in 
contact.      Every   molecule   is  ceaselessly  vibrating 
to   and   fro   among    its   neighbors,   often    hitting 
them  and  rebounding  from  them. 

10.  Physical  Science. — Physical   science   com- 
prises Physics  and  Chemistry. 

Physics  deals  with  masses  and  molecules;  chemistry, 
with  atoms  and  combinations  of  atoms. 

Experiment  7. — Bring  a  piece  of  warm  sealing  wax  near  some 
small  bits  of  paper,  but  without  touching  them.  Notice  that  the 
paper  bits  do  not  move.  Briskly  rub  the  sealing  wax  with  a  piece 
of  warm  flannel  and  quickly  bring  it  near  the  paper  bits  as  before. 
Notice  that  the  sealing  wax  now  produces  very  peculiar  motions  in 


8  NATURAL   PHILOSOPHY.  §  II 

the  paper  bits.     Notice  also  that  you  have  wrought  an  important 
change  in  the  sealing  wax,  but  that  still  the  stick  is  sealing  wax. 

Experiment  8.— Rub  a  brass  button  on  the  floor  or  carpet  until 
it  is  uncomfortably  warm.  Notice  that  while  you  have  produced  a 
change  in  the  button,  it  is  still  brass;  you  did  not  change  the 
substance  of  which  it  is  made. 

11.  What  is  a  Physical  Change?—.^  physical 
change  is  one  that  does  not  change  the  nature 
of  the  molecule. 

(a.)  A  piece  of  marble  may  be  ground  to  powder,  but  each  grain 
is  marble  still.  Ice  may  change  to  water  and  water  to  steam,  yet 
the  identity  of  the  substance  is  unchanged.  A  piece  of  glass  may 
be  electrified  and  a  piece  of  iron  magnetized,  but  they  still  remain 
glass  and  iron.  These  changes  alter  the  distances  and  arrange- 
ment of  the  molecules,  but  leave  the  molecules  themselves  un- 
changed ;  they  are  physical  changes.  A  change  like  the  rusting 
of  iron  or  the  burning  of  wood,  changes  the  nature  of  the  molecule 
itself,  and,  consequently,  of  the  substance.  Stick  a  change  as  this 
is  called  a  chemical  change. 

12.  What   is    a  Property   of   Matter?  —  Any 
quality  of  matter  is  called  a  property  of  matter. 

Lead  is  heavy ;  heaviness  is  a  property  of  lead.  It 
may  be  manifested  without  changing  the  lead  to  any- 
thing else,  and  is,  therefore,  called  a  physical  property. 
Sulphur  or  brimstone  is  brittle.  The  brimstone  may 
axhibit  this  property  and  still  remain  brimstone.  Hence, 
brittleness  is  a  physical  property  of  brimstone. 

Sulphur  is  also  combustible— it  may  be  burned.  But 
this  property  of  sulphur  (combustibility)  can  not  be 
shown  without  changing  the  sulphur  to  something  else. 
Such  are  called  chemical  properties  of  mqtter* 


§  14  DEFINITION.  9 

13.  Definition. — Physics,  or  Natural  Philosophy, 
is  the  branch  of  science  that  treats  of  the   laws 
and  physical   properties  of  matter,  and  of  those 
phenomena  that  depend  upon  physical  changes. 

14.  Recapitulation.— To  be  reproduced  and  ampli- 
fied by  the  pupil  for  review. 


«  2 

w     S 
>•     i 


10  NATURAL  PHILOSOPHY.  §  15 

SECTION     II. 
THE    PHYSICAL    PROPERTIES    OF    MATTER. 

15.  Division  of  Physical  Properties.—  The  phys- 
ical properties  of  matter  are  divided  into  two  classes, 
universal  and  characteristic. 

16.  What    are    Universal    Properties  ?  —  Uni- 
versal properties  of  matter  are  such  as  belong  to 
matter  of  every  kind. 

17.  List   of  Universal    Properties.  —  The  prin- 
cipal  universal   properties   of    matter  are    extension, 
impenetrability,  weight,  indestructibility,  inertia, 
divisibility,     porosity,     compressibility,     expansi- 
bility and  elasticity. 

18.  What   is   Extension  ?  — Extension  is  that 
property  of  matter   by   virtue   of  which   it  takes 
up  room. 

It  is  involved  in  the  definition  of  matter  given  in  §  1. 

(a.)  In  this  country  and  in  England,  the  foot  and  yard  are  units 
commonly  used  at  the  present  time.  But  in  most  other  civilized 
countries  of  the  world,  the  international  or  metric  units  are  used. 
These  units  are  almost  universally  adopted  hy  scientific  men  even  in 
England  and  America. 

In  this  book,  frequent  use  will  be  made  of  the  terms  meter, 
liter  and  gram,  their  multiples  and  divisions.  The  pupil  should 


§  l8      THE  PHYSICAL  PROPERTIES   OF  MATTER.          11 


familiarize  himself  with  these  units.     He  will  find  further  informs 
tion  concerning  them  in  Appendix  B.  at  the  end  of  this  volume. 

Experiment  9.— Pass  a  bent  tube  and  a  funnel,  as  shown  in 
Fig.  1,  or  a  funxxel  tube  as  shown  in  Fig.  2, 
through  the  cork  of  a  bottle.  Be  sure 
that  alt  joints  are  air  tight.  The  delivery 
tube  is  best  marte  of  glass  which  may  be 
bent  when  headed  to  redness  in  an  alco- 
hol or  gas  flaine.  Place  the  end  of  the 
delivery  tube  in  a  tumbler  of  water. 
Pour  water  through  the  funnel.  As  it 
runs  into  the  bottle,  air  will  be  forced 
out  and  may  be  seen  bubbling  through 
the  water  in  the  tumbler.  A  bottle  con- 
venient for  this  and  other  experiments  may  be  prepared  by  per- 
forating the  cover  of  a  glass  fruit  jar,  as 
shown  in  Fig.  2.  The  holes  carry  cork  or 
caoutchouc  stoppers,  through  which  the 
tubes  pass.  Full  directions  for  bending 
glass  tubing,  boring  holes,  etc.,  may  be 
found  in  Appendix  4,  of  Chemistry. 


FIG.  1. 


JIMi 


FIG.  2. 


Experiment  10. — Thrust  a  lamp  chimney 
into  water.  The  water  will  rise  inside  the 
chimney,  entering  at  the  lower  end  and 
pushing  the  air  out  at  the  top.  Repeat  the 
experiment,  closing  the  upper  end  of  the 
chimney  with  the  hand  (or  use  an  inverted 
tumbler).  The  water  can  not  rise  as 
before  because  the  vessel  is  filled  with  air 
that  can  not  escape. 


Experiment  II. — Drive  a  nail  into  a  piece 
of  wood  ;  the  particles  of  wood  are  either  crowded  more  closely 
together  to  give  room  for  the  nail,  or  bume  of  them  are  driven 


12  NATURAL  PHILOSOPHY.  §  19 

oat  before  it.     Clearly,  the  iron  and  the  wood  are  not  in  the 
same  place  at  the  same  time. 

19.  What    is    Impenetrability  ?  —  Impenetra- 
bility  is   that   property  of   matter    by   virtue    of 
which   two  bodies  can  not  be  in  the  same   place 
at  the  same  time. 

20.  What  is  Weight  ? — Weight  is  the  measure 
of  mass  attraction  or  gravity. 

(a.)  The  word  gravity  generally  points  out  the  tendency  ot 
bodies,  not  supported,  to  fall  to  the  ground. 

(6.)  A  body  is  heavy  or  light  according  to  the  amount  of  attrac- 
tion ;  the  greater  the  attraction,  the  greater  the  weight. 

(c.)  If  an  apple  be  held  in  the  hand,  the  attraction  between  the 
earth  and  the  apple  produces  pressure  upon  the  hand.  This  pres- 
sure is  not  the  attraction,  but  it  is  the  measure  of  it  and  is  called 
weight. 

(d.)  If  the  same  apple  were  upon  the  moon,  its  weight  would 
be  the  measure  of  the  attraction  between  the  apple  and  the  moon. 
But  as  the  moon  has  less  matter  than  the  earth,  the  attraction 
between  the  apple  and  the  moon  would  be  less  than  that  between 
the  apple  and  the  earth  and  the  weight  would,  consequently,  be 
less. 

21.  What    is    Indestructibility?  —  Indestructi- 
bility  is   that    property   of   matter   by   virtue   of 
which  it  can  not  be  destroyed. 

(a)  No  human  being  can  create  or  destroy  a  single  atom  of 
matter.  Water  evaporates  and  disappears  only  to  be  gathered  in 
clouds  and  condense  and  fall  as  rain.  Wood  burns,  but  the  ashes 
and  smoke  and  the  invisible  gases  formed,  contain  the  identical 


§  22  tNERTTA.  13 

atoms  of  which  the  wood  was  composed.  In  a  different  form,  the 
matter  still  exists  and  weighs  as  much  as  before  it  was  burned. 
The  experiment  is  difficult,  but  has  been  repeatedly  performed. 
The  universe  contains  the  same  atoms  to-day  that  it  did  at  th« 
close  of  the  creation— not  one  more,  not  one  less. 

Experiment  12. — Upon  the  tip  of  the  fore-finger  of  the  left 
hand,  place  a  common  calling-card.  Upon  this  card  and  directly 
over  the  finger,  place  a  cent. 
With  the  nail  of  the  middle 
finger  of  the  right  hand,  let  a 
sudden  blow  or  "  snap "  be 
given  to  the  card.  A  few  trials 
will  enable  you  to  perform  the 
experiment  so  as  to  drive  tlu 
card  away  and  leave  the  coin 
resting  upon  the  finger.  The 
card  flies  away  on  account  of 

the  force  of  the  "  snap."  The  cent  remains  on  the  finger  because 
the  blow  is  so  quick  that  the  card  has  no  time  to  give  any  of 
its  motion  to  the  coin. 

Experiment  13. — To  n.ake  the  experiment  still  more  interest 
ing,  use  a  bullet  instead  of  the  cent  and  the  open  top  of  a  bottle 
instead  of  the  finger-tip.  Keep  trying  until  you  succeed  in  drop 
ping  the  bullet  into  the  bottle.  The  experiment  illustrates  the 
inertia  of  the  bullet,  the  card  being  driven  away  before  it  lias 
opportunity  to  impart  its  motion  to  the  bullet. 

22.  What  is  Inertia?  —  Inertia  is  that  prop- 
erty of  matter  by  virtue  of  ichich  it  has  a  ten- 
dency ivhen  at  rest  to  remain  at  rest  or  when 
in  motion  to  continue  in  motion. 

(a.)  A  ball  cannot  put  itself  in  motion.  When  the  ball  is  thrown 
through  the  air,  it  has  no  power  to  stop  and  it  will  not  stop  until 
Borne  external  force  compels  it  to  do  so.  This  external  force  may 


14  NATURAL  PHILOSOPHY.  §  22 

be  the  bat,  the  catcher,  the  resistance  of  the  air  or  the  force  of 
gravity.  It  must  be  something  outside  the  ball  or  the  ball  will 
move  on  forever. 

(&.)  Illustrations  of  the  inertia  of  matter  are  so  numerous  that 
there  should  be  no  difficulty  in  getting  a  clear  idea  of  this  property. 
The  "running  jump"  and  "  dodging  "  of  the  playground,  the  fre- 
quent falls  which  result  from  jumping  from  cars  in  motion,  the 
hackward  motion  of  the  passengers  when  a  car  is  suddenly  started 
and  their  forward  motion  when  a  car  is  suddenly  stopped,  the  diffi- 
culty in  starting  a  wagon  and  the  comparative  ease  of  keeping  it  in 
motion,  etc.,  etc.,  may  be  used  to  illustrate  this  property  of  matter. 

Experiment  14. — Strike  a  piece  of  loaf  sugar  or  of  brick  with  a 
hammer.  The  sugar  or  brick  is  separated  into  many  parts. 

23.  What  is  Divisibility  ?  —  Divisibility  is  that 
property  of  matter  by  virtue  of  ivhich  a  body 
may  be  separated  or  divided  into  parts. 

(a)  The  divisibility  of  matter  may  be  carried  to  such  an  extend 
as  to  excite  our  wonder  and  test  the  powers  of  imagination  itself. 
It  is  said  that  the  spider's  web  is  made  of  threads  so  fine  that 
enough  of  this  thread  to  go  around  the  earth  would  weigh  but 
half  a  pound  and  that  each  thread  is  composed  of  six  thousand 
filaments.  A  single  inch  of  this  thread  with  all  its  filaments  ma;r 
be  cut  into  thousands  of  distinct  pieces  and  each  piece  of  each 
filament  be  yet  a  mass  of  matter  composed  of  molecules  and  atoms 
We  may  consider  that  the  atom  marks  the  limit  of  divisibility. 

Experiment  15.— Fill  a  test  tube  with  water.  Slowly  add 
sugar.  A  considerable  quantity  may  be  added  without  increasing 
the  bulk  of  the  liquid. 

Experiment  16.— Take  a  test  tube  three  quarters  full  of  water 
and  carefully  add  alcohol  until  it  is  filled.  Close  the  tube  with  the 
thumb  and  shake.  Notice  that  the  tube  is  no  longer  full.  We 
know  that  the  water  and  the  alcohol  cannot  be  in  the  same  plac« 


§  25  COMPRESSIBILITY,  15 

at  the  same  time  and  are  forced  to  the  conclusion  that  some  of 
the  water  molecules  have  been  received  into  the  space 
between  the  alcohol  molecules  or  vice  versa. 

24.  What  is  Porosity  ?— Porosity  is  that  prop- 
erty of  matter  by  virtue   of  which  spaces   exist 
between  the  molecules. 

(a.)  When  iron  is  heated,  the  molecules  are  pushed  further 
apart,  the  pores  are  enlarged  and  we  Bay  that  the  iron  has 
expanded.  If  a  piece  of  iron  or  lead  be  hammered,  it  will  bo 
made  smaller  because  the  molecules  are  forced  nearer  together, 
thus  reducing  the  size  of  the  pores. 

(6. )  These  pores  are  very  large  in  comparison  with  the  size  of 
the  molecules.  As  was  said  a  few  pages  back,  if  a  race  of  persons 
could  be  imagined  small  enough  to  live  on  a  molecule  as  we  live 
on  our  earth,  we  might  fancy  them  looking  across  the  space  around 
it  and  seeing  the  nearest  molecule  as  we  look  off  into  the  sky  and 
see  the  moon  or  stars. 

(c.)  Cavities  or  cells,  like  those  of  bread  and  sponge,  are  some- 
times improperly  spoken  of  as  pores. 

25.  What  is  Compressibility? — Compressibility 
is  that  property  of  mat- 
ter by  virtue  of  u-hich 

a  body  may  be  reduced 
in  size. 

Experiment  17.  — Invert  a 
thin  glass  bottle  over  a  plate  of 
water,  as  shown  in  Fig.  4.  The 
heat  of  the  hand  will  expand 
the  air  in  the  bottle  and  some 
of  it  will  escape  in  bubbles. 
If  no  bubbles  appear,  pour  warm 
water  over  the  bottle. 


16  XATtfoAL  PtitLOSOPXT.  §  26 

26.  What  is  Expansibility  ?  —  Expansibility  is 
that   property   of  matter   by   virtue   of  which   a 
body  may  be   increased  in  size. 

(a.)  Compressibility  and  expansibility  are  the  opposites  of  each 
other,  resulting  alike  from  porosity.  Let  each  pupil  prove,  by 
experiment  with  an  ordinary  pop-gun  or  other  apparatus,  that  air 
Is  compressible  and  expansible. 

Experiment  18. — Provide  strips  of  rubber,  whalebone,  wood, 
iron,  steel,  brass,  copper,  zinc  and  lead.  Stretch  the  piece  of  rubber 
and  notice  what  takes  place  when  the  stretching  force  ceases  to 
act.  Bend  each  of  the  other  strips  and  notice  what  takes  place 
under  similar  circumstances. 

27.  What   is   Elasticity  ?  —  Elasticity   is   that 
property    of   matter    by    virtue    of   which    bodies 
resume   their   original  form   or   size    ivhen   that 
form   or   size  has  been  changed  by  any  external 
force. 

All  bodies  possess  this  property  in  some  degree.  Solid 
bodies  have  elasticity  of  form;  liquids  and  gases  have 
not. 

(a.)  Different  substances  possess  this  property  in  different 
degrees. 

Solid  bodies  are  not  perfectly  elastic.  They  may  contract  after 
being  stretched  or  expand  after  being  compressed  but  they  do 
not  return  to  exactly  their  former  size. 

Liquids  and  gases  are  perfectly  elastic.  No  matter  how  great 
the  pressure  that  may  be  exerted  upon  them,  they  return  to 
exactly  their  former  size  when  the  pressure  is  removed. 

The  ordinary  spring-balance  owes  its  value  to  the  elasticity  of 
the  spiral  steel  spring,  which  is  stretched  by  the  weight. 


§  30  COHESION  AND  ADHEStON.  I? 

28.  What    are    Characteristic    Properties?  — 
Characteristic   properties   of  matter   are  such  as 
belong  to  matter  of  certain  kinds  only. 

They  enable  us  to  distinguish  one  substance  from 
another.  Glass  is  brittle,  and  by  this  single  property 
may  be  distinguished  from  India-rubber. 

29.  List    of   Characteristic    Properties.  —  The 

characteristic  properties  of  matter  are  numerous.  They 
depend  chiefly  upon  cohesion  and  adhesion.  The  most 
important  are  hardness,  tenacity,  brittleness,  mal- 
leability and  ductility. 

Experiment  19.— Take  a  sheet  of  gold-leaf  in  your  fingers  and 
try  to  pick  the  metal  off  with  the  fingers  of  the  other  hand. 
Some  of  the  gold  will  "  stick  to  your  fingers." 

30.  What  are  Cohesion  and  Adhesion?  —  Co- 
hesion is  the  force  that  holds  together  like  mole- 
cules;  adhesion  is  the  force   that   holds  together 
unlike  molecules. 

(a.)  Cohesion  is  the  form  of  molecular  attraction  that  holds  most 
substances  together  and  gives  them  form.  If  you  pull  on  an  iron 
wire  and  are  unable  to  break  it,  you  are  to  understand  that  the 
cohesion  of  the  iron  particles  is  stronger  than  you  are. 

(6.)  Adhesion  is  the  form  of  molecular  attraction  that  causes  the 
penci/  or  crayon  to  leave  marks  upon  the  paper  or  blackboard  and 
makes  paste,  glue,  mortar  and  cements  "  stick." 

(c.)  In  a  brick  wall,  cohesion  binds  together  the  molecules  of  the 
mortar  layer  into  a  single,  hardening  mass  ;  adhesion  reaches  out 
and  grasps  the  adjoining  bricks  and  holds  them  fast — a  solid  wall. 
Each  acts  only  through  distances  too  small  to  be  measured. 


18  NATURAL  PHILOSOPHY.  §  31 

Experiment  20.— Get  pieces  of  chalk,  glass,  iron,  lead,  copper 
and  marble.  Try  to  scratch  each  one  with  each  of  the  others. 
Make  a  note  of  the  result  of  each  experiment,  thus  : 

Glass  will  scratch  copper  ;  copper  will  not  scratch  glass.  Deter- 
mine which  is  the  hardest  substance  in  your  collection  and  which 
is  the  softest. 

31.  What    is    Hardness?  —  Hardness   is   that 
property    of    matter    by    virtue    of    which    some 
substances  resist  any  attempt  to  force  a  passage 
between  their  particles. 

It  is  measured  by  the  degree  of  difficulty  with  which 
one  substance  is  scratched  by  another.  Fluids  are  not 
said  to  have  hardness. 

32.  What  is  Tenacity  ?— Tenacity  is  that  prop- 
erty  of   matter   by    virtue    of   which   some    sub- 
stances resist  a  force  tending  to   pull   their  par- 
ticles asunder. 

(a.)  Like  hardness  and  the  other  characteristic  properties  of 
matter,  it  is  a  variety  of  cohesion  which  is  the  general  term  for 
the  force  which  holds  the  molecules  together  and  keeps  masses 
from  crumbling  into  dust. 

33.  What  is  Brittleness  t—Brittleness  is  that 
property    of    matter    by    virtue    of    which    some 
substances  may  be  easily  brolcen,  as  by  a  blow. 

(a.)  Glass  furnishes  a  familiar  example  of  this  property.  The 
idea  that  brittlenoss  is  the  opposite  of  hardness,  elasticity  or 
tenacity,  should  be  guarded  against  Glass  is  harder  than  wood, 
but  very  brittle :  it  is  very  elastic,  but  very  brittle  also.  Steel  is 
far  more  tenacious  than  lead  and  far  more  brittle. 


§  36  RECAPITULATION.  If 

34.  What    is  Malleability  ?  —  Malleability    it 
that  property  of  mutter  by  virtue  of  which  some 
substances    may  be    rolled    or    hammered    into 
sheets. 

(a.)  Gold  is  the  most  malleable  metal.  It  has  been  beaten  so 
thin  that  a  pile  of  282,000  leaves  would  be  but  an  inch  high. 

Experiment  21.— Heat  the  middle  of  a  piece  of  glass  tubing 
about  six  inches  long,  in  an  alcohol  flame,  until  red-hot.  Roll  the 
ends  of  the  glass  slowly  between  the  fingers  and,  when  the  heated 
part  is  soft,  quickly  draw  the  ends  asunder.  That  the  fine  glass 
wire  thus  produced  is  still  a  tube,  may  be  shown  by  blowing 
through  it  into  a  glass  of  water  and  noticing  the  bubbles  that 
will  rise  to  the  surface. 

35.  What  is  Ductility  ? — Ductility  is  that  prop- 
erty    of   matter   by   virtue   of   which    some   sub- 
stances may  be  drawn  into  wire. 

(a.)  Platinum  wire  has  been  made  OTF<TT  of  an  inch  in  diameter 
Glass,  when  heated  to  redness,  is  very  ductile,  as  was  shown  in 
the  last  experiment.  All  ductile  substances  are  tenacious,  but  a 
tenacious  substance  is  not  necessarily  ductile. 

36.  Recapitulation. — To  be  reproduced  and  ampli- 
fied by  the  pupil  from  memory. 


PHYSICAL 


GENERAL. 


Extension,  Impenetrability,  Weight 
Indestructibility,  Inertia,  Divisi 
bility,  Porosity,  Compressibility 


Expansibility,  Elasticity. 

PROPERTIES 


OF   MATTER. 

(  ADHESION.       Tenacity. 
CHARACTERISTIC.  \  „  \  Brittle*e*s. 

\  COHESION.      MatteaMity. 
[  Ductility 


20  NATURAL  fjfiLosoPalr.  §  3? 

SECTION     III. 
THE   THREE   CONDITIONS   OF    MATTER. 

37.  Conditions   of  Matter.  —  Matter  exists  in 
three  conditions  or  forms — the   solid,  the  liquid; 
and  the  aeriform. 

Ice  is  solid ;  water  is  liquid ;  steam  is  aeriform. 

38.  What  is  a  Solid  ? — A  solid  is  a  body  whose 
molecules  move  among  themselves  with  difficulty. 

Such  bodies  have  a  strong  tendency  to  retain  any  form 
that  may  be  given  to  them.  A  movement  of  one  part  of 
such  a  body  produces  motion  in  all  of  its  parts. 

Experiment  22.— Place  your  finger  in  a  vessel  of  water  and 
move  it  about.  The  watery  particles  easily  flow  over  and  around 
one  another ;  there  is  great  freedom  of  molecular  motion.  Remove 
your  finger,  holding  the  tip  downward.  The  water  molecules  in- 
istantly  glide  into  the  space  lately  occupied  by  your  finger,  which 
leaves  no  hole  behind  it.  Notice  that  a  drop  of  water  hangs 
upon  your  finger  tip.  That  drop  contains  many  molecules  which 
cling  together,  held  by  the  force  of  cohesion,  while  the  drop  clings 
to  your  finger,  held  by  the  force  of  adhesion. 

Experiment  23. — Suspend  a  glass  or  metal  plate,  of  about  four 
inches  area,  from  one  end  of  a  scale-beam  and  accurately  balance 
the  same  with  weights  in  the  opposite  scale-pan.  The  support- 
ing cords  may  be  fastened  to  the  plate  with  wax.  Beneath  the 
plate,  place  a  saucer  so  that  when  the  saucer  is  filled  with  watel 


§  40  AERIFORM  BODY.  21 

the  plate  may  rest  upon  the  liquid  surface,  the  scale  beam  remain- 
ing horizontal.  Carefully  add 
small  weights  to  those  in  the 
scale- pan.  Notice  that  the  water 
beneath  the  plate  is  raised  above 
its  level.  Add  more  weights 
until  the  plate  is  lifted  from 
the  water.  Notice  that  the 
under  surface  of  the  plate  is 
wet.  These  water  molecules  on 
the  plate  have  been  torn  from 
their  companions  in  the  saucer. 
The  added  weights  were  needed 

to  overcome  the  tendency  of  the  water  molecules  to  cling  to- 
gether. 

NOTE.— After  seeing  a  physical  experiment,  always  ask  your- 
self  "  What  was  the  object  of  that  experiment  ?  What  does  it 
teach?"  Never  allow  yourself  to  look  upon  an  experiment  as 
being  simply  entertaining  ;  thus  reducing  the  experimenter,  so  far 
as  you  are  concerned,  to  the  level  of  a  showman. 

39.  What  is  a  Liquid?  —  A  liquid  is  a  body 
whose  molecules  easily  move  among  themselves, 
yet  tend  to  cling  together. 

Liquids  adapt  themselves  to  the  form  of  the  vessel 
containing  them  but  do  not  retain  that  form  when  the 
restraining  force  is  removed.  They  always  so  adapt 
themselves  as  to  have  their  free  surfaces  horizontal. 
Water  is  the  best  type  of  liquids. 

40.  What   is   an   Aeriform   Body?  —  An  ae'ri- 
form  body  is  like  air.     Its  molecules  easily  move 
among    themselves    and    tend    to    separate   from 
each  other  almost  indefinitely. 


22  NATURAL  PHILOSOPHY.  §40 

A  vessel  may  be  half  full  of  a  solid  or  a  liquid  but  not 
of  an  aeriform  substance.  Atmospheric  air  is  the  best 
type  of  aeriform  bodies.  Aeriform  means  "having  the 
form  of  air." 

Experiment  24. — Place  n  piece  of  ice  in  a  large  metal  spoon. 
Hold  the  bowl  of  the  spoon  in  the  flame  of  a  lamp.  Notice  that 
the  solid  ice  changes  to  liquid  water  and  finally  disappears  as  a 
vapor. 

Experiment  25. — Heat  half  a  brick  in  the  stove,  place  it  on  any 
convenient  support,  drop  a  few  scales  of  iodine  (which  you  can  get 
at  the  chemist's)  upon  the  brick  and  cover  the  brick  with  a  large 
bell  glass.  The  glass  will  quickly  be  filled  with  the  beautiful 
violet  colored  vapor  of  iodine.  Notice  whether  the  iodine  changes 
to  a  liquid  before  it  becomes  a  vapor,  as  the  ice  did. 

41,  Gases  and   Vapors. — Aeriform  bodies  are  of 
two  kinds,  gases  and  vapors. 

Gases  remain  aeriform  under  ordinary  conditions, 
although  they  may  be  changed  to  the  liquid  form  by 
intense  cold  and  pressure.  Oxygen  is  a  gas. 

Vapors  are  produced  by  heat  from  substances  that 
are  generally  solid  or  liquid.  They  resume  the  solid  or 
liquid  form  at  ordinary  temperatures.  Steam  is  a  vapor. 

42.  Changes  of  Condition. — The  same  substance 
may  exist  in  two  or  even  three  of  these  forms.     Most 
solids,  as  lead  and  iron,  may  be  changed  by  heat  to 
liquids  ;   others,  as  iodine,  may  be  apparently  changed 
directly  to  vapors ;    still  others,  as  ice,  may  be  easily 
changed  first  to  the  liquid  and  then  to  the  vapor  form. 
It  is  probable  that  our  present  inability  to  liquefy  and 
vaporize  certain  substances  arises  from  our  limited  means 


§  43  FLUID.  23 

for  the  production  of  heat  Many  substances  that 
formerly  could  not  be  melted  are  easily  melted  in  the  arc 
of  the  now  common  electric  lamp.  With  this  idea  in 
mind,  we  may  say  that  a  solid  is  frozen  matter;  that  a 
liquid  is  melted  matter ;  that  a  gas  is  vaporized  matter. 

43.  Ultra-Gaseous  Form  of  Matter.  —  Recent 
experiments  with  electric  discharges  in  high  vacuums 
[§§  187,  239,  306,]  have  yielded  remarkable  results 
which,  in  the  opinion  of  many,  prove  the  existence  of 
a  fourth  condition  of  matter.  For  matter  in  this  ex- 
tremely thin  or  attenuated  form,  the  name  "Radiant 
Matter "  has  been  proposed. 

(a.)  In  a  very  remarkable  lecture  on  this  subject  (August  22, 
4879),  Prof.  COOKES  said  :— 

"  Gases  are  considered  to  be  composed  of  an  almost  infinite  num- 
ber of  molecules,  which  are  constantly  moving  in  every  direction 
with  velocities  of  all  conceivable  magnitudes.  As  these  molecules 
are  exceedingly  numerous,  it  follows  that  a  molecule  can  not 
move  far  in  any  direction  without  coming  into  contact  with  some 
other  molecule.  But  if  we  exhaust  the  gas  contained  in  a  closed 
vessel,  the  number  of  molecules  becomes  diminished  and  the  dis- 
tance through  which  any  one  of  them  can  move  without  coming 
into  contact  with  another  is  increased.  The  mean  free  path  is 
inversely  proportional  to  the  number  of  molecules  present.  The 
further  the  exhaustion  is  carried,  the  longer  becomes  the  average 
distance  a  molecule  can  travel  before  entering  into  collision.  By 
thus  lengthening  the  mean  free  path  of  the  remaining  molecules, 
we  obtain  phenomena  so  distinct  from  anything  which  occurs  in  air 
or  gas  at  the  ordinary  tension  that  we  are  led  to  assume  that  we 
are  here  brought  face  to  face  with  matter  in  a  fourth  state  or  con- 
dition, one  as  far  removed  from  the  state  of  a  gas  as  a  gas  is  from 
a  liquid." 


NATURAL   PHILOSOPHY. 


§44 


44-  What  is  a  Fluid?  —  A  fluid  is  a  body 
whose  molecules  easily  change  their  relative  po- 
sitions. 

The  term  includes  liquids,  gases  and  vapors. 

45.  Recapitulation. — To  be  reproduced,  upon  paper 
or  the  blackboard,  by  eaoh  pupil. 


MATTER. 


\  SOLIDS. 

Molecules  change 

their  relative  po- 

sitions with  diffi- 

culty 

LIQUIDS. 

Molecules  cling  to- 

gether feebly. 

FLUIDS. 

GASES  ;     ordinar 

Molecules  change 
their  relative  po- 
sitions easily. 

AERIFORM  BODIES. 
Molecules    tend    to 
separate. 

rily  aeriform. 

VAPORS  ;  ordina- 
rily liquid  o: 
tfli£. 

§  45  QUESTIONS   FOR   REVIEW.  25 


QUESTIONS     FOR     REVIEW. 

1.  How  should  the  "  Book  of  Nature  "  be  read? 

2.  (a.)  What  term  is  applied  to  anything  that  you  can  see,  feel, 
touch  or  taste  ?    (6.)  What  is  energy  ? 

3.  What  limits  the  number  of  eleme^r.  ° 

4.  A  stone  that  measures  eight  cubic  inches  is  quietly  placed  in 
a  bowl  full  of  water.     How  much  water  will  run  out  ?    Why  ? 

5.  What  is  an  element  ? 

6.  Given  a  lamp  chimney,  a  small  cork  and  a  pail  of  water; 
how  will  you  illustrate  the  compressibility  of  air  ? 

7.  (a.)  What  is  the  smallest  possible  division  of  an  element? 
(6.)  Of  a  compound  substance? 

8.  Two  hydrogen  atoms  make  a  hydrogen  molecule.     Is  hydro- 
gen an  elementary  or  a  compound  substance  ? 

9.  Two  hydrogen  atoms  and  one  oxygen  atom  make  a  water 
molecule.     Is  water  an  elementary  or  a  compound  substance  ? 

10.  (a.)  If  you  thrust  a  knitting  needle  into  a  mass  of  dough,  is 
the  hole  thus  made  a  pore?     (b.)  What  is  a  pore ? 

11.  Are  the  molecules  of  water  larger  or  smaller  than  those  of 
steam  ? 

12.  Name  two  classes  of  fluids  ? 

13.  Give  an  illustration  of  molecular  motion  in  a  mass  that  is  at 
rest. 

14.  Which  are  the  greater,  the  diameters  of  molecules  or  the 
distances  between  molecules  ? 

15.  Are  intermolecular  spaces  greater  in  water  or  in  steam? 

16.  What  is  molecular  attraction  called  ? 

17.  Give  an  illustration  (not  contained  in  this  book)  of  one  of 
the  universal  properties  of  matter. 

18.  What  is  the  difference  between  a  fluid  and  a  liquid  ? 

19.  One  sixteen  thousandth  of  a  cubic  inch  of  indigo  dissolved 
in  fuming  sulphuric  acid  will  give  a  perceptible  color  to  two  or 
three  gallons  of  water.     What  property  of  matter  may  thus  be 
illustrated? 


CHAPTER    U. 

MOTION    AND    FORCE.— DYNAMICS.- 
GRAVITATION,  ETC.— ENERGY. 


SECTION     I. 

MOTION    AND    FORCE. 

46.  What   is  Motion?  —  Motion  is  a  changing 
of  position. 

No  body  can  move  or  be  moved  from  one  place  to 
another  without  motion. 

47.  What  is   Force?  —  As  generally  used,  force 
signifies  any  cause  that  tends  to  produce,  change, 
or  destroy  motion.    All  physical  phenomena  are  caused 
by  the  action  of  forces  upon  matter. 

(a.)  There  are  many  kinds  of  force.  We  often  hear  and  speak 
of  the  force  of  muscular  action,  the  force  of  the  wind,  the  force 
of  gravity,  etc.  We  shall  soon  see  that  heat,  light,  magnetism, 
electricity,  etc.,  may  exercise  force. 

48.  What   is   Dynamics  ?  —  Dynamics   is   that 
branch  of  Natural  Philosophy  or  Physics  which 
treats  of  forces  and  their  effects. 

(a.)  The  word  "mechanics"  was  formerly  used  in  the  sense 
that  we  now  use  the  word  "dynamics."  Mechanics  properly 
denotes  the  science  of  machines  and  is  a  branch  of  dynamics. 


§  49  MOTION  AND  FORCE.  27 

Experiment  26.— Place  on  the  floor  a  croquet  ball  and  a  heavj 
iron  ball.  Strike  them  equal  blows  with  a  mallet.  Notice  that 
equal  forces  do  not  always  produce  equal  velocities. 

Experiment  27.  —Roll  the  iron  ball  and  the  croquet  ball  with 
equal  velocities.  Find  out  which  requires  the  greater  force  to 
stop  its  motion. 

49.  Momentum. — The  momentum  of  a  body  is 
its  quantity  of  motion. 

(a.)  Momentum  depends  upon  the  weight  of  the  moving  body 
and  its  velocity  or  rapidity  of  motion. 

Examples.— An  iceberg  moves  very  slowly  but  almost  irresisti- 
bly, or  with  great  momentum,  because  it  is  tfry  Jieary.  A  ballet 
is  not  very  heavy  but,  when  fired  from  a  rifle,  it  has  a  great  mo- 
mentum because  it  moves  with  great  Telocity. 

(6.)  Momentum  is  generally  measured  by  the  product  of  the 
numbers  repi  3senting  the  weight  and  the  velocity.  The  unit  of 
momentum  has  no  definite  name. 

M=  W  x  V. 

Examples. — The  momentum  of  a  body  having  a  weight  of  80 
pounds  and  a  velocity  of  15  feet,  is  twice  as  great  as  that  of  a  body 
having  a  weight  of  5  pounds  and  a  velocity  of  30  feet.  The  mo- 
mentum of  the  former  is  300  ;  that  of  the  latter,  150. 

(<?.)  In  comparing  momenta,  the  pupil  must  be  careful  that  the 
units  of  weight  used  are  alike.  The  units  of  velocity  also  must 
be  alike.  If  the  weight  of  one  body  be  given  in  ounces  and  that 
of  the  other  in  pounds,  they  must  be  reduced  to  the  same  denomina- 
tions. If  the  velocity  of  one  body  lie  given  in  feet  per  minute  and 
that  of  the  other  in  miles  per  hour,  a  similar  reduction  must  be 


28  NATURAL  PHILOSOPHY.  §  50 

50.  Laws  of  Motion.— The  following  propositions, 
known  as  Newton's  Laws  of  Motion,  are  so  important 
and  so  famous  that  they  ought  to  be  remembered  by 
every  pupil. 

(1.)  Every  body  continues  in  its  state  of  rest 
or  of  uniform  motion  in  a  straight  line 
unless  compelled  to  change  that  state  by 
an  external  force. 

(2.)  Every  motion  or  change  of  motion  is  in 
the  direction  of  the  force  impressed  and 
is  proportionate  to  it. 

(3.)  Action  and  reaction  are  equal  and  oppo- 
site in  direction. 

51.  The   First   Law.  — This  is  called  the  law  of 
inertia,  because  it  results  directly  from  inertia  (§  22). 
It  is  impossible  to  furnish  perfect  examples  of  this  law, 
because  all  things  within  our  reach  or  observation  are 
acted  upon  by  some  external  force.     A  base-ball  when 
once  set  in  motion  has  no  power  to  stop  itself.     If  its 
motion  was  not  interfered  with,  it  would  move  in   a 
straight  line  but  the  force  of  gravity  is  ever  active, 
turns  it  from  that  line  and  compels  it  to  move  in  a 
graceful  curve  instead. 

Experiment  28.— Rapidly  whirl  a  pail,  partly  filled  with  water, 
in  a  vertical  circle.  Not  a  drop  will  fall  from  the  bucket  even 
when  it  is  upside  down  because  the  water  seeks  to  move  in  a 
straight  line  and  thus  to  move  away  from  the  center  about 
which  it  is  swung. 


§52 


MOTION  AND   FORCE. 


52.  Centrifugal  Force.— Although  it  is  impossible 
to  give  any  experimental  proof  of  the  first  law  of  motion, 
we  sec  many  illustrations  of  the  tendency  of  moving 
bodies  to  move  in  straight  lines  even  when  forced  to 
move  in  curves.  Examples,  such  as  water  flying  from  a 


Fio.  6. 

revolving  grindstone  or  mud  from  a  carriage-wheel,  arc 
familiar  to  all.  A  wagon  in  rapidly  turning  a  corner  is 
likely  to  be  overturned  for  the  same  reason.  This  fact 
explains  why  the  outer  rail  on  a  railway  curve  is  laid 
higher  than  the  inner  one.  A  stone  is  thus  shot  from  a 
sling. 

The  school-boy  is  sent  rolling  in  the  game  of  "  crack- 
the-whip  "  because,  while  ilie  mentalpart  of  the  boy  may 


SO  NATURAL  PHILOSOPHY.  $  52 

seek  to  move  in  a  curve  in  obedience  to  the  pull  of  hig 
leader,  the  material  part  of  the  boy  seeks  to  move  in  a 
straight  line  in  obedience  to  Newton's  First  Law  of 
Motion.  The  fun  of  the  game  arises  from  the  fact  that 
the  matter  often  triumphs  over  the  mind. 

This  tendency  of  matter  to  move  in  a  straight 
line  and,  consequently,  further  away  from,  the 
center  around  which  it  is  revolving,  is  called 
Centrifugal  Force. 

(a.)  The  laws  of  centrifugal  force  may  be  studied  or  illustrated 
by  the  whirling  table  and  accompanying  apparatus,  represented  in 
Fig.  6. 

53.  The  Second  Law. — The  second  law  of  motion 
is  sometimes  given  as  follows : 

A  given  force  will  produce  the  same  effect 
whether  the  body  on  which  it  acts  is  in  motion 
or  at  rest;  whether  it  is  acted  on  by  that  force 
alone  or  by  others  at  the  same  time. 

(a.)  If  a  ball  that  is  moving  with  a  velocity  of  50  feet  a  second 
be  hit  with  a  force  that,  acting  alone,  would  produce  a  velocity 
of  25  feet  a  second,  the  ball  will  have  a  velocity  of  75  feet  a 
second. 

(6.)  If  a  boat  be  pulled  northward  with  a  force  that  would  give 
it  a  velocity  of  3  miles  per  hour  and,  at  the  same  time,  pulled 
eastward  with  a  force  that  would  give  it  a  velocity  of  4  miles  per 
hour,  it  will  move  nearly  northeastward  and  with  a  velocity  of 
5  miles  per  hour. 

At  the  end  of  the  hour,  the  boat  will  be  at  the  place  where  it 
would  be  if  the  first  force  had  really  moved  it  3  miles  northward 
and  the  second  force  had  then  moved  it  4  miles  eastward. 


§  54  MOTION  AND  FORCE.  31 

Experiment  29.— Float  upon  water  two  blocks  of  wood,  one  of 
which  is  twice  as  heavy  as  the  other.  Connect  them  by  a  stretched 
rubber  cord.  Release  the  blocks  and  they  will  move  toward  each 
other,  but  with  unequal  velocities.  Determine  how  much  faster 
one  moves  than  the  other,  and  compare  their  momenta. 

54.  The  Third  Law. — Examples  of  the  third  latf 
of  motion  are  very  common.  When  we  strike  an  egg 
upon  a  table,  the  action  of  the  egg  may  make  a  dent  in 
the  table,  while  the  reaction  of  the  table  breaks  the  egg. 

The  oarsman  urges 
the  water  backward  with 
the  same  force  that  he 
urges  his  boat  forward. 

In  springing  from  a 
boat  to  the  shore,  mus- 
cular action  tends  to 
drive  the  boat  adrift ; 
the  reaction,  to  put  the 
passenger  ashore. 

Experiment  30.— Hang 
two  clny  balls  of  equal 
wsight  by  strings  of  equal 
lengths  so  that  they  will 
just  touch  each  other.  If 
ouo  be  drawn  aside  and  let 
fall  against  the  other,  both 
will  move  forward,  but 
only  half  as  far  as  the  first 
would  have  moved  had  it 
met  no  -esistance.  FIG.  7. 


62  NATURAL  PHILOSOPHY.  §  54 

Experiment  31. — Place  two  ivory  balls,  which  are  elastic,  as 
you  did  the  clay  balls  of  Experiment  30.  Repeat  the  experiment. 
The  first  ball  will  give  the  whole  of  its  motion  to  the  second 
and  remain  still  after  striking,  while  the  second  will  swing  about 
as  far  as  the  first  would  have  done  if  it  had  met  no  resistance. 
In  this  case,  as  in  the  former,  it  will  be  seen  that  the  first  ball 
loses  as  much  momentum  as  the  second  gains. 

Experiment  32.  —  Make  a  railway  of  two  wooden  strips, 
1-|  inches  by  \  inch  and  about  six  feet  long,  fastened  together  by 
three  or  five  cross-pieces,  as  shown  in  Fig.  8.  The  distance 
between  the  rails  should  be  about  an  inch.  Place  the  railway  on 
a  board  and  fasten  down  the  middle  cross  piece  with  a  screw. 
Spring  up  the  ends  and  support  them  by  books  or  wooden  blocks. 
At  the  toy  shop,  get  several  large  glass  "  marbles  "  and  place  them 
on  the  middle  of  the  railway.  Bring  one  ball  to  the  highest  point 
of  the  track  and  let  it  roll  down  against  the  others.  Ball  No.  1 
gives  up  its  motion  to  No.  2  and  comes  to  rest  ;  No.  2  gives  it  to 
No.  3  and,  in  turn,  comes  to  rest.  The  energy  is  thus  passed 
through  the  line  -to  No.  7,  which  is  driven  some  distance  on 


^"Ar-,  i:;,,:: 

76      f 


FIG.  8. 


the  up  grade,  as  to  the  position  shown  by  the  dotted  line  at  8. 
From  8,  this  ball  rolls  down  grade  and  passes  its  energy  along  the 
/ine,  forcing  No.  1  up  the  grade  to  a  lesser  distance  than  before. 
The  balls  will  repeat  their  motions  several  times  until  they  are 
finally  brought  to  rest  by  friction,  etc. 


§57 


MOTION  AND  FORCE. 


33 


Experiment  33.— This  action  of  ivory  or  glass  balls  is  due  t« 
the  fact  that  they  are  elastic,  and 
are  flattened  by  the  blow.  To 
show  that  this  is  so,  smear  a  flat 
stone  or  iron  plate  with  paint. 
Before  the  paint  becomes  dry, 
ylace  one  of  the  glass  balls  on  the 
•meared  surface  and  notice  the 
«ize  of  the  round  spot  thus  made. 
Then  drop  the  ball  from  a  height 
of  several  inches  and  notice  that 
the  spot  made  is  larger  tJuin  before. 
This  shows  that  the  glass  ball  was 
flattened  just  as  truly  as  if  it  were 
made  of  rubber.  Elasticity  at 
once  restores  the  original  shape. 


FIG.  9. 


55.  Effect  of  Elasticity  upon    Reaction.— The 

effects  of  action  and  reaction  are  largely  modified  by 
elasticity,  but  never  so  as  to  destroy  their  equality. 

56.  Reflected  Motion.—  Reflected  motion  is  the 
motion    produced    by   the  reaction  of   a  surface 
when  struck  by  a  body,  the  surface,  or  the  bo*111 
or  both  being  elastic. 

A  ball  rebounding  from  the  wall  of  a  house   is  an 
example  of  reflected  motion. 

57.  Law   of  Reflected  Motion.  — The  angle  in- 
cluded between  the  line  in  which  the  body  moves  before 
it  strikes  the  reflecting  surface  and  a  perpendicular  to 
.that  surface  drawn  from  the  point  of  contact,  is  called 
the  angle  of  incidence.     The  angle  between  the  line 


NATURAL   PHILOSOPHY. 


§57 


in  which  the  body  moves  after  striking  and  the  perpen- 
dicular, is  called  the  angle  of  reflection. 


FIG.  10. 

The  angle  of  incidence  is  equal  to  the  angle 
cf  reflection. 

A  ball  shot  from  A  will  be  reflected  at  B  back  to  C, 
making  the  angle  of  incidence  ABD,  equal  to  the  angle 
cf  reflection,  0  B  D. 


58.  Recapitulation.— To  be  amplified  by  the  pupil 
for  review. 


DYNAMICS. 


FORCE. 


DEFINITION. 

FORMS. 

"CENTRIFUGAL." 


MOMENTUM. 
MOTION.    \  NEWTON'S  LAWS. 

REFLECTED   MOTION. 


§  58  EXERCISES. 


EXERCISES. 

1.  What  is  the  momentum  of  a  100-pound  ball  moving  275  fee* 
a  second  ?  Ans.  27500. 

2.  A  25-pound  ball  is  moving  100  feet  a  second.     A  two-pountf 
bird  is  flying  at  the  rate  of  50  feet  A  second.    The  momentum  ol 
the  ball  is  how  many  times  as  great  as  ths«t  of  the  bird  ? 

Am.  25  timea 

3.  A  50- pound  body  has  a  momentum  of  1000.     What  r'lmbei 
will  represent  its  velocity  ?  Ans.  20. 

4.  Which  has  the  greater  momentum,  a  steamboat  at  rest  or  r 
canoe  in  motion  ?    Why  ? 

5.  A  boat  that  is  moving  at  the  rate  of  5  miles  an  hour  weigta 
4  tons;    another  that  is  moving  at  the  rate  of  10  miles  an  hour 
weighs  2  tons.     How  do  their  momenta  compare  ? 

6.  A  stone  weighing  12  ounces  is  thrown  with  a  velocity  of  23 
feet  a  second .     An  ounce  ball  is  shot  with  a  velocity  of  15  miles  a 
minute.     Which  will  have  the  greater  momentum?    How  many 
times  as  great  ?  Ans.  5. 

7.  Can  an  angle  of  incidence  be  greater  than  a  right  angle  ? 

8.  (a.)  When  water  is  heated  it  becomes  steam.      Is  this  a 
physical  or  a  chemical  change?     (6.)   When  steam  is  intensely 
heated  it  is  changed  into  a  mixture  of  two  gases,  oxygen  and 
hydrogen.     This  mixed  gas  will  not  condense  to  water,  but    will 
burn  and  even  explode.     What  kind  of  a  change  is  this?  (c.)  What 
was  divided  in  the  latter  case  that  was  not  in  the  former  ? 

9.  Bend  a  twig  and  tell  what  change  is  thus  wrought  upon  the 
molecules  on  the  convex  side  of  the  twig  and  what  upon  (hose  on 
the  concave  side  ? 

10.  (a.)  Why  are  pile  drivers  made  very  heavy?    '(6.)  Why  are 
they  raised  to  considerable  heights? 

11.  A  man  weighing  ICO  pounds  stands  in  a  boat  that  weighs 
1000  ]x>unds  and  pulls   on  a  rope  held  by  a  man  weighing  200 
pounds  and  standing  in  a  boat  weighing  2000  pounds.     Compare 
the  velocities  and  momenta  of  the  two  boats. 


36  NATURAL  PHILOSOPHY.  §  59 

SECTION      II. 

GRAVITATION. 

59.  What  is  Gravitation  ?  —  Every  particle  of 
matter  in    the    universe    has    an    attraction  for 
every    other    particle.      This    attractive    force    is 
called  gravitation. 

60.  Laws  of  Gravitation.  —  (1.)  Gravitation  va- 
ries directly  as  the  product  of  the  masses. 

For  example,  doubling  either  mass  doubles  the  attrac- 
tion ;  trebling  either  mass  will  multiply  the  attraction 
by  three. 

(2.)  Gravitation  varies  inversely  as  the  square 
of  the  distance  between  the  centres  of  gravity 
(§  65). 

For  example,  doubling  the  distance  quarters  the  at- 
traction ;  trebling  the  distance  will  divide  the  attraction 
by  nine. 

Doubling  both  the  product  of  the  masses  and  the 
distance,  will  halve  the  attraction  ;  trebling  both  the 
product  and  the  distance  will  divide  the  attraction  by 


61.  What  is  Gravity  ?  —  Gravity  is  th  attrac- 
tion between  the  earth  and  bodies  upon  or  near 
its  surface. 


§64  GRAVITATION.  37 

It  is  the  kind  of  gravitation  with  which  we  are  most 
familiar.     It  acts  in  a  vertical  di-    . 
rection,  as  shown  by  the  plumb 
line,  Fig.  11. 

62.  What  is  Weight?  — 
Weight  is  the  measure  of 
gravity. 


The  attraction  between  two 
pounds  of  iron  and  the  earth  is 
twice  as  great  as  the  attraction 
between  one  pound  of  iron  and 
the  earth.  Because  its  gravity  is 
twice  as  great,  we  say  that  its 


Fio.  11. 


Weight  is  twice  as  great  or  that  it  is  twice  as  heavv. 

63.  Law   of  Weight.  —  Bodies   weigh  most   at 
tne  surface  of  the  earth. 

Below  the  surface,  the  weight  decreases  as  the 
distance  to  the  centre  of  the  earth  decreases. 

Jlbovc  the  surface,  the  weight  decreases  as  the 
square  of  the  distance  from  the  centre  of  the 
earth  increases. 

64.  Examples.  —  How  much  will  a  pound  of  iron 
weigh  one  thousand  miles  below  the  surface  of  the  earth  ? 

Ans. — As  the  iron  would  then  be  only  three-fourths 
»s  far  from  the  earth's  centre,  it  would  weigh  only  three- 
fourths  as  much,  or  twelve  ounces. 


67361 


38  NATURAL  PHILOSOPHY.  §  64 

How  far  below  the  surface  of  the  earth  will  a  ten-pound 
ball  weigh  only  four  pounds  ? 

Ans. — As  it  is  to  weigh  only  f  as  much,  its  distance 
from  the  earth's  centre  must  be  only  f  of  the  earth's 
radius  :  £  of  4,000  miles  is  1,600  miles.  It  will  be  1,600 
miles  from  the  centre  or  2,400  miles  from  the  surface. 

A  body  at  the  earth's  surface  weighs  900  pounds  : 
What  would  it  weigh  8,000  miles  above  the  surface  ? 

Ans. — It  would  then  be  12,000  miles  from  the  earth's 
centre  or  three  times  as  far.  The  square  of  three  is 
nine.  According  to  the  law  above  given,  it  would 
weigh  only  £  as  much,  or  100  pounds. 

Experiment  34. — Six  inches  from  the  upper  end  of  a  string 
about  two  feet  long,  tie  a  small  loop.  Fasten  any  convenient 
weight,  as  an  apple  or  large  key,  to  the  lower  end  of  the  string. 
Stick  two  stout  pins  or  tacks  into  adjacent  corners  of  the  frame  of 
your  slate.  Slip  the  loop  over  one  pin  and  support  the  slate  by  the 
short  part  of  the  string,  allowing  the  weight  to  hang  free.  When 
the  slate  and  string  have  come  to  rest,  mark  a  line  on  the  slate, 
showing  exactly  the  position  of  the  string.  Support  the  slate  in  a 
similar  manner  from  the  other  pin  and  draw  another  line  on  the 
slate,  again  showing  exactly  the  position  of  the  string.  Place  your 
finger  at  the  point  where  these  two  lines  on  your  slate  cross  each 
other  and  balance  the  slate  horizontally  upon  your  finger  tip. 

65.  Centre  of  Gravity.  —  The  centre  of  gravity 
of  a  body  is  the  point  about  which  all  the  mat- 
ter composing  the  body  may  be  balanced. 

A  body  thus  balanced  is  said  to  be  "in  equilibrium." 
The  weight  of  a  body  may  be  supposed  to  be  concen- 
trated at  the  centre  of  gravity. 


§66 


GRAVITATION. 


be  Outside 


66.  The  Centre  of  Gravity  may 
of  the  Body. — The  centre  of  grav- 
ity may  be  outside  of  the  matter 
of  which  a  body  consists,  as  in  the 
case  of  a  ring,  hollow  sphere,  box, 
or  cask.  The  same  fact  is  illustrated 
by  the  "balancer,"  represented  in 
Fig.  12.  The  centre  of  gravity  is  in 
the  line  joining  the  two  heavy  balls, 
MM,  and  thus  under  the  foot  of  the 
waltzing  figure. 


(a.)  The  "balancer"  may  be  bought  at 

the  toy  store  for  a  few  cents.     The  pupil 

may  better  make  one,  using  a  large  cork  for 

the  body  and  a  smaller  one  for  the  head. 

The  neck,  arms  and  legs  may  be  made  of 

Btout  pins,  parts  of  hair-pins  or  other  wire. 

The  heavy  balls  may  be  two  small  potatoes.  .  The  uplifted  arm 
may  be  made  of  extra  length  and  serve  as  a 
staff  for  a  paper  flag,  neatly  colored  with  red 
and  blue  ink  or  pencil.  Instead  of  using 
the  heavy  balls,  the  prongs  of  two  forks  or 
the  open  blades  of  two  penknives,  as  shown 
in  Fig.  13,  may  be  thrust  into  the  cork,  the 
only  thing  necessary  being  to  bring  the  centre 
of  gravity  lower  down  than  the  foot  of  the 
figure.  It  is  far  better  for  the  pupil  to  mnk« 
his  apparatus,  when  he  can,  than  to  buy  it. 
He  will  understand  more  dearly,  learn  mort 
rapidly,  and  soon  enjoy  the  exercise  of  his  in- 

gem.ity.     The  remark  applies  to  girls  as  well   as  to  boys,  and 

in  many  classes  it  happens  that  the  girls  are  more  ingenious  and 

enterprising  than  the  bov^ 


Fio 


40  NATURAL  PHILOSOPHY.  §  6"/ 

67.  Equilibrium.  —  When  the  centre  of  gravity 
is  supported,  the  whole  body  will   rest  in   a  state 
of  equilibrium. 

The  centre  of  gravity  will  be  supported  when  it  and 
the  point  of  support  are  at  the  same  place  or  in  the 
same  vertical  line. 

(a.)  A  yard-stick  may  be  supported  by  a  needle  thrust  through 
its  middle ;  the  point  of  support  and  the  centre  of  gravity  are 
together. 

(6.)  It  may  be  balanced  with  one  end  resting  on  the  finger  ;  the 
point  of  support  and  the  centre  of  gravity  are  in  the  same  vertical 
line,  the  latter  being  directly  over  the  former. 

(e.)  It  may  be  supported  by  hanging  it  by  a  string ;  the  point  of 
support  and  the  centre  of  gravity  are  again  in  the  same  vertical 
line,  the  latter  now  being  directly  under  the  former. 

(d.)  The  yard-stick  in  these  three  positions  illustrates  the  three 
conditions  or  kinds  of  equilibrium. 

68.  Stable  Equilibrium. — A  body  supported  in 
such  a  way  that,  when  slightly  displaced  from 
its  position  of  equilibrium,  it   tends  to  return  to 
that  position,  is  said  to  be  in  stable  equilibrium, 

Such  a  displacement  raises  the  centre  of  gravity. 
Examples :  a  disc  or  round  flat  plate  supported  above  the 
centre;  a  semi-spherical  oil-can;  a  pendulum  or  plumb- 
line.  The  cavalry-man  represented  in  Fig.  14  may  rock 
up  and  down  balanced  upon  his  horse's  hind-feet,  because 
the  heavy  ball  brings  the  centre  of  gravity  of  the  com- 
bined mass  below  the  points  of  support.  The  "  balancer" 
(Fig.  12)  affords  another  example  of  stable  equilibrium. 


§70  GRAVITATION.  4i 

(a.)  This  apparatus  is  easily  made.  A  piece  of  board  an  inch 
thick  may  be  fashioned  into  a 
shape  something  like  the  body 
of  a  horse.  The  head  and 
neck  may  be  made  of  paste- 
board. A  feather  makes  a 
good  tail  and  small  nails 
answer  admirably  for  legs. 
A  potato  or  apple  may  be 
used  for  the  heavy  ball.  The 
support  may  be  made  by  plac- 
ing a  lath  across  th3  backs  of 
two  chairs.  The  "  balancer  " 
previously  made  may  ride  this 
bare-backed  horse  standing 
wherever  he  is  placed,  the  °' 

weight  of  the  apple  and  the  length  or  curvature  of  the  wire  being 
varied  to  suit  the  circumstances. 

69.  Unstable   Equilibrium. — A  body  supported 
in    such    a    way    that,    ivhen    slightly    displaced 
from  its  position  of  equilibrium,  it  tends  to  fall 
further  from  that   position,  is  said  to  be  in  un- 
stable equilibrium. 

Such  a  displacement  lowers  the  centre  of  gravity. 
The  body  will  not  come  to  rest  until  the  centre  of 
gravity  has  reached  the  lowest  possible  point,  when  it 
will  be  in  stable  equilibrium.  Examples  :  A  disc  sup- 
ported below  its  centre ;  an  egg  standing  on  its  end ; 
a  stick  balanced  upright  upon  the  finger. 

70.  Neutral  Equilibrium. — A  body  supported  in 
such  a  way  that,  when  displaced  from  its  posi- 
tion of  equilibrium,  it  tends  neither  to  return  tq 


42  NATURAL  PHILOSOPHY.  §  JQ 

its  former  position  nor  to  fall  further  from  it, 
is  said  to  be  in  neutral  or  indifferent  equi- 
librium. 

Such  a  displacement  neither  raises  nor  lowers  the 
centre  of  gravity.  Examples :  A  disc  supported  at  its 
centre  ;  a  sphere  resting  on  a  horizontal  surface. 

71.  Line  of  Direction. — *4  vertical  line  drawn 
downward  from  the  centre  of  gravity  is  called 
the  line  of  direction. 

It  may  be  considered  as  a  line  connecting  the  centre 
of  gravity  of  the  given  body  and  the  centre  of  the  earth. 

72.  The    Base.  —  The  side  on  which  a  body 
rests  is  called  its  base. 

If  the  body  be  supported  on  legs,  as  a  chair  or  table, 
the  base  is  the  figure  formed  by  joining  the  points  of 
support. 

73.  Stability.  —  When    the   line  of   direction 
falls  within  the  base,  the  body  stands;   when 
without  the  base,  the  body  falls. 

When  the  body  rests  upon  a  point,  as  does  the  sphere., 
or  upon  a  line,  as  does  the  cylinder,  a  very  slight  force  is 
sufficient  to  move  it,  no  elevation  of  the  centre  of  gravity 
being  necessary. 

The  broader  the  base  and  the  lower  the  centre 
of  gravity,  the  greater  the  stability. 

(a.)  These  facts  explain  the  stability  of  leaning  towers  like  those 
of  Pisa  and  Bologna.  In  some  such  towers  the  centre  of  gravity 
is  lowered  by  using  heavy  materials  for  the  lower  part  and  light 
materials  for  the  upper  part  of  the  structure. 


§74 


GRAVITATION. 


43 


(6.)  It  is  difficult  to  stand  upon  one  foot  or  to  walk  upon  a  tight 
rope  because  of  the  smallness  of  the  base. 


PIG.  15. 

(c.)  A  porter  carrying  a  pack  is  obliged  to  lean  forward  :  a  man 
Carrying  a  load  in  one  hand  is  obliged  to  lean  away  from  the  load, 
to  keep  the  common  centre  of  gravity  of  man  and  load  over  the 
base  formed  by  joining  the  extremities  of  his  feet. 

74.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review 


f  DEFINITION. 


GRAVITATION. 


GRAVITY 


DEFINITION. 

WEIGHT {S"''"*' 

CENTRE  OF  GRAVITY. -j  n-L*re 

(  Stable. 

EQUILIBRIUM -(Unstable. 

(  Neutral. 


44  NATURAL  PHILOSOPHY.  §  74 


EXERCISES. 

1.  Why  can  a  child  walk  more  easily  witl-      ane  than  without  1 

2.  Why  will  a  book  placed  on  a  desk-lici  stay  there  while  a 
marble  would  roll  off  ? 

3.  Why  is  a  pyramid  a  very  stable  form  of  structure  ? 

4.  Why  is  a  ton  of  stone  on  a  wagon  less  likely  to  upset  than  a 
ton  of  hay  similarly  placed  ? 

5.  If  the  weight  of  a  body  be  doubled,  what  will  be  the  effect 
on  its  attractive  force  ? 

6.  If  two  bodies  attract  each  other  with  a  force  of  four  units, 
what  will  be  their  attractive  force  when  the  distance  between  them 
is  doubled? 

7.  How  far  does  the  earth's  attraction  extend? 


§75 


FALLING  BODIES. 


SECTION      III. 

FALLING    BODIES. 

Experiment  35. — Drop  a  feather  and  a  cent  at  the  same  tim« 
from  the  same  height  and  notice  that  the  cent  will  reach  tin 
ground  first. 


Experiment  36. — Take  an  iron  and 
a  wooden  ball  of  the  same  size,  drop 
them  at  the  same  time  from  an  upper 
window  and  notice  that  they  will 
strike  the  ground  at  nearly  the  same 
time.  You  will  find  that  it  requires  a 
little  practice  to  drop  them  at  exactly  the 
tame  time. 

75.  Velocities  of  Falling 
Bodies. — In  Experiment  35,  the 
cent  fell  faster  than  the  feather, 
because  it  met  with  less  resist- 
ance from  the  air  than  the  feather 
did  and  not  because  it  was  heavier. 

In  Experiment  30,  we  made  this 
resistance  of  the  air  nearly  equal 
and,  probably,  convinced  ourselves 
that  the  velocity  of  a  falling  body 
does  not  depend  upon  its  weight, 
unless  it  has  to  overcome  resist- 
ance, or  perform  work,  while  it 
is  falling. 


FIG.  1G. 


46  NATURAL  PHILOSOPHY.  §  75 

When  the  resistance  of  the  air  is  removed,  the  feather 
and  the  cent  will  fall  with  equal  velocities.  This  resist- 
ance may  be  avoided  by  trying  the  experiment  in  a  glass 
tube  from  which  the  air  has  been  removed.  The  experi- 
ment is  difficult  to  perform  and  requires  expensive 
apparatus.  Fig.  16  shows  the  cent  and  the  feather 
falling  with  equal  velocities  in  a  long  tube  from  which 
the  air  has  been  removed  with  an  air  pump,  an  instru- 
ment that  we  shall  soon  consider.  §  187. 

76.  Reason  of  this  Equality. — The  cent  is  heavier 
than  the  feather  and  is,  therefore,  pulled  downward  by  a 
greater  force.    The  irovi  ball  has  the  greater  weight,  which 
shows  that  it  is  acted  upon  by  a  greater  force  than  the 
wooden  ball.      But  this  greater  force  has  to  move  a 
greater  load,  has  to  do  more  work  than  the  lesser  force. 

For  the  greater  force  to  do  the  greater  work 
requires  as  much  time  as  for  the  lesser  force  to 
do  the  lesser  work. 

A  regiment  can  march  no  further  in  an  hour  than  a 
single  soldier  can  ;  a  thousand  molecules  can  fall  no 
further  in  a  second  than  a  single  molecule  can. 

77.  Gravity  is  a  Constant  Force.— In  these  ex- 
periments, the  feather,  the  cent  and  the  balls  could  not 
change  from  their  condition  of  rest  to  that  of  motion  by 
their  own  power  (§  22)*.     It  was  necessary  that  some 
force  act  upon  them  to  produce  their  motion.    The  force 
of  gravity  is  what  did  the  work. 

During  the  first  second,  the  force  of  gravity  gave  the 
falling  ball  a  certain  velocity ;  it  gave  the  ball  just  as 


§  79  FALLING  BODIES.  4? 

much  more  velocity  during  the  next  second  and  just  aa 
mucli  more  during  the  third  second.  At  the  end  of  the 
third  second,  the  ball  was  moving  just  three  times  as  fast 
as  it  was  at  the  end  of  the  first  second. 

</l  force  that  thus  continues  to  net  uniformly 
upon  a  body,  even  after  the  body  has  begun 
to  move,  is  called  a  constant  force.  Gravity  is 
a  constant  force. 

If  gravity  or  any  other  constant  force  gives  a  body  a 
velocity  of  32  feet  in  one  second,  it  will  give  a  velocity 
of  64  feet  in  two  seconds  and  a  velocity  of  96  feet  in 
three  seconds.  The  increase  of  velocity  is  32  feet  in 
each  second. 

78.  Freely  Falling  Bodies.— When  a  falling  body 
meets  with  no  resistance,  it  is  called  a  freely  falling 
body.     For  heavy  bodies,  the  resistance  of  the  air  is  so 
little  that  it  is  generally  left  out  of  the  account.     A  ball 
rolling  down  an  inclined  plane,  can  not  be  considered  a 
freely  falling  body. 

(a.)  The  laws  of  freely  falling  bodies  have  been  very  carefully 
studied.  The  apparatus  used  for  this  purpose  is  somewhat  expen 
sive.  It  is  carefully  described  in  the  section  on  "  Falling  Bodies ' 
In  the  author's  Elements  of  Natural  Philosophy. 

79.  Laws  of  Falling  Bodies. — These  laws  are  a? 
follows : 

(1.)  The  velocity  of  a  freely  falling  body  at 
the  end  of  any  second  of  its  descent  is 
equal  to  32.16  feet  or  9.81  meters  mul- 
tiplied by  the  number  of  the  second. 


48  NATURAL  PHILOSOPHY.  §  79 

(2.)  The  distance  traversed  by  a  freely  fall- 
ing  body  during  any  second  of  its  de- 
scent is  equal  to  16.08  feet  or  Jj.,9 
meters  multiplied  by  one  less  than 
twice  the  number  of  seconds. 

(3.)  The  distance  traversed  by  a  freely  fall- 
ing body  during  any  number  of  sec- 
onds is  equal  to  16.08  feet  or  Jf.9 
meters  multiplied  by  the  square  of  tht 
number  of  seconds. 

80.  Initial  Velocity  of  Falling  Bodies.  —  We 
iave  been  considering  bodies  falling  from  a  state  of  rest, 
gravity  being  the  only  force  producing  the  motion.  But 
a  body  may  be  thrown  downward  as  well  as  dropped. 
In  such  a  case,  the  effect  of  the  throw  must  be  added  to 
the  effect  of  gravity.  If  a  body  be  thrown  downward 
with  an  initial  or  starting  velocity  of  fifty  feet  per  sec- 
ond, we  must  add  fifty  feet  to  the  result  obtained  under 
the  first  or  second  laws  above  or  fifty  feet  multiplied  by 
the  number  of  the  seconds  to  the  result  obtained  under 
the  third  law. 


81.  Increment  of  Gravity.  —  Since  gravity 
the  velocity  of  a  freely  falling  body  32.  1C  feet  or  9.81 
meters  each  second,  this  quantity  is  called  the  increment 
of  velocity  due  to  gravity  or,  more  simply,  the  increment 
of  gravity.  In  physical  mathematics,  it  is  generally 
represented  by  the  letter  g.  It  must  be  remembered 
that  g  =  32.16  feet  or  9.81  meters. 


FALLING 


49 


82.  Formulas. — If  we  represent  the  velocity  at  the 
end  of  any  second  by  v,  the  number  of  seconds  by  t,  the 
distance  passed  over  each  second  by  s,  a,',  d  the  total  space 
fallen  through  by  S,  we  shall  have  the  following  formulas 
for  freely  falling  bodies  : 


With  50  feet  initial  velocity. 

v  =  gt  +  50. 

s  =  \g  (%t  —  1)  +  50. 

rr 


With  no  initial  'velocity. 
(1.)  v=gtorlg  x 
(2.)   s  =  $g(2t-l). 
(3.)  S=lgt*. 


83.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


IMPEDED  ;    VELOCITIES  VARY. 

(  DEFINITION. 


FALLING    BODIES. 


FREELY  FALLING.- 


(  VELOCITIES  EQUAL. 


GRAVITY. 


( A  CONSTANT  FORCE. 


(  CAUSES  INCREMENT  OF  VELOCITY 
EFFECTS  OF  INITIAL  VELOCITY. 
LAWS  AND  FORMULAS. 


50  NATURAL  PHILOSOPHY.  §  83 


EXERCISES. 

1.  How  far  will  a  body  fall  in  10  seconds?          Ant.  1608  ft, 

2.  What  velocity  will  a  freely  falling  body  attain  in  4  seconds' 

Ana.  128.64  ft. 

3.  How  far  will  a  body  move  during  the  fourth  second  of  itg 
fall?  Ar,».  112.56ft. 

4.  How  far  will  a  body  move  during  the  fifth  second  of  its  fall, 
if  it  starts  with  a  velocity  of  25  feet  per  second  ? 

5.  (a.)  If  a  body  rolling  down  an  inclined  plane  gains  a  velocity 
of  10  feet  in  the  first  second,  what  will  be  its  velocity  at  the  end 
of  the  tenth  second  ?    (6.)  What  is  its  increment  of  velocity  ? 

Am.  (a.)  100ft. 

6.  (a.)  What  is  the  increment  of  gravity  in  meters?     (6.)  In 
centimeters  ? 

7.  Two  balls  are  dropped,  one  3  seconds  after  the  other.    When 
the  second  one  has  fallen  for  two  seconds,  how  far  is  it  from  the 
first?  Ant.  387.68ft. 

8.  A  falling  body  has  a  velocity  of  98.1  meters.     How  long  has 
it  been  falling?  Ans.  10  sec. 

9.  Show  that  the  formula,  S  =  i  gt*.  given  in  §  82,  is  the  sam« 
in  meaning  as  the  third  law  given  in  §  79. 


§  85  *HE  PENDULUM.  51 

SECTION      IV. 
THE    PENDULUM. 

84.  The  Pendulum. — A  common  pendulum  is 
a  weight  so  suspended  as  to  be  capable  of  swing- 
ing to  and  fro. 

It  appears  in  many  forms.  The  most  common  form 
consists  of  a  steel  rod,  thin  and  flexible  at  the  top,  carry- 
ing at  the  bottom  a  heavy  mass  of  metal  known  as  tn*, 
bob. 

85.  Motion  of  the  Pendulum.  —  When  the  sup- 
porting thread  or  bar  of  the  pendulum  is  vertical,  the 
centre  of  gravity  is  in  the  lowest  possible  position.     The 
pendulum  then  remains  at  rest,  for  the  force  of  gravity 
tends  to  draw  it  downward,  thus  producing  pressure  at 
the  point  of  support  but  no  motion.     When  the  pendu- 
lum is  drawn  from  its  vertical  position,  the  centre  of 
gravity  is  raised.     Gravity  draws  the  pendulum  back  to 
a  vertical  position,  when  inertia  carries  it  beyond  until 
it  is  stopped  and  drawn  back  again  by  gravity.     It  thus 
swings  to  and  fro  in  an  arc. 

Experiment  37. — In  an  open  doorway  or  other  convenient 
place,  suspend  a  weight  by  a  string  or  fine  wire,  four  or  five  feet 
long.  Swing  this  pendulum  through  an  arc  about  a  foot  long 
and  count  the  vibrations  that  it  will  make  in  a  minute.  Then 
swing  the  same  pendulum  through  an  arc  two  or  three  feet  long 
and  again  count  the  vibrations  that  it  will  make  in  a  minute. 


NATURAL    PHILOSOPHY. 


86 


FIG.  17. 


86.  First  Law  of  the  Pendu- 
lum.—  The  vibrations  of  a  given 
pendulum,  at  any  given  place, 
are  performed  in  equal  times 
whether  the  arc  be  long  or  short. 

Experiment  38. — Provide  another  pendu- 
lum of  the  same  length  as  the  one  used  in  the 
last  experiment,  but  make  it  considerably 
heavier  or  considerably  lighter.  Remember 
that  the  true  length  of  a  pendulum  is  the 
distance  between  the  point  of  support  and  a 
point  near  the  centre  of  gravity,  which  latter 
point  is  called  the  centre  of  oscillation.  Set 
both  pendulums  in  vibration  at  the  same  time 
and  notice  whether  the  heavy  or  the  light  one 
vibrates  the  more  rapidly. 


87.  Second  Law  of  the  Pendulum.  —  The  time 
of  vibration  is  independent  of  the  weight  or  ma- 
terial of  the  pendulum,  depending  only  upon  the 
length  of  the  pendulum  and  the  intensity  of  the  force  of 
gravity  at  any  given  place. 

Experiment  39. — Prepare  two  pendulums  of  such  lengths  that 
one  shall  vibrate  just  twice  as  often  as  the  other.  One  will  be 
several  times  as  long  as  the  other.  Find  just  liow  many  times  as 
long  it  is. 

Then  prepare  two  pendulums,  one  of  which  shall  vibrate  just 
three  times  as  often  as  the  other  and  determine  the  ratio  between 
their  lengths  as  before. 

Do  the  same  thing  with  two  pendulums,  one  of  which  vibrates 
four  times  as  fast  as  the  other.  Place  the  ratios  that  you  have 
found  in  the  places  of  x,  y  and  z  in  the  following  table : 


§  89  THE  PENDULUM. 


Numbers  of  Vibrations.  Lengtht. 

1 1 

2 * 

3 y 

4 z 

5 f 

6 f 

Can  you  see  any  law  or  rule  governing  in  such  cases?  Try,  with- 
out  experiment,  to  put  the  proper  numbers  in  the  places  of  the 
two  interrogation  points.  Notice  that  the  greater  the  length, 
the  less  the  number  of  vibrations. 

88.  Third  Law  of  the  Pendulum.  —  The  vibra- 
tions of  pendulums  of  different  lengths  are  performed  in 
different  times.      The  lengths  are  inversely  propor- 
tional  to   the  squares   of  the   numbers  of  vibra* 
tions  in  a  given  time. 

L  :  I  ::  n*  :  JV». 

89.  The  Second's  Pendulum.— The  length  of  a 
second's  pendulum,  at  the  level  of  the  sea,  is  39 
inches   at   the    equator;    39.2    inches,   near   the 
poles    and    about    39.1    inches    or    993.3    milli- 
meters  or  .9933  meters,  in  this  latitude. 

As  such  a  pendulum  would  be  inconveniently  long, 
use  is  generally  made  of  one  one-fourth  as  long  which, 
consequently,  vibrates  half  seconds. 

The  length  and  time  of  vibration  of  the  second's 
pendulum  being  thus  known,  the  length  of  any  other 
pendulum  may  be  found  when  the  time  of  vibratipo  is 


54  NATURAL  PHILOSOPHY.  §  89 

given.     The  time  of  vibration  may  be  found  when  the 
Length  is  given. 

(a.)  The  third  law  may  be  used  in  solving  such  a  problem.  It 
is  interesting  to  notice  how  little  difference  there  is  between  the 
length  of  a  second's  pendulum  and  the  meter. 

90.  Use  of  the  Pendulum  in  Time-pieces. — The 

motion  of  a  clock  is  due  to  the  force  of 
gravity  acting  upon  the  weights,  or  to 
the  elasticity  of  the  spring.  But  the 
weights  have  a  tendency  toward  accel- 
erated motion  (increasing  velocity), 
while  the  spring  would  give  an  exam- 
pie  of  diminishing  motion.  Either 
defect  would  be  fatal  in  a  time-piece. 
Hence,  the  properties  of  the  pendulum 
set  forth  in  the  first  and  third  laws  are 
used  to  regulate  this  motion  and  make 
it  available  for  the  desired  end. 

If  the  clock  gains  time,  the 
pendulum  is  lengthened  by  low- 
ering the  bob ;  if  it  loses  time,  the 
pendulum  is  shortened  by  raising 
the  bob. 

Remember  that  the  pendulum  does 
not  make  the  clock  go  ;  it  simply  de- 
termines how  fast  it  shall  run.    It  does 
'FIG   18  ^n*s  ^7  means  °f  the  escapement,  shown 

in   Fig.   18.      Every  vibration  of  the 

l«endulnm  works  the  crutch,  n  m,  and  allows  the  wheel, 

/?,  to  more  forward  one  tooth, 


§91 


THE  PENDULUM. 


55 


91.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


DEFINITION. 

CAUSE  OF  VIBRATIONS. 


THE    PENDULUM. 


LAWS 


f  FIRST. 
SECOND. 


{  THIRD. 

THE  SECOND'S  PENDULUM. 
APPLICATION  TO  CLOCKS. 


56  NATURAL  PHILOSOPHY.  §  QI 


EXERCISES. 

1.  If  a  pendulum  swings  through  an  arc  two  feet  long,  how 
many  more  times  will  it  vibrate  in  a  minute  than  when  it  swings 
through  an  arc  four  feet  long  ? 

2.  One  pendulum  is  10  inches  long  and  vibrates  four  times  as 
fast  as  another.     What  is  the  length  of  the  other? 

3.  Two  pendulums  are  of  the  same  length.     The  bob  of  one 
weighs  a  pound  ;  that  of  the  other,  half  a  pound.     Compare  their 
rates  of  vibration. 

4.  One  pendulum  is  16  inches  long ;  another  is  64  inches  long. 
How  many  times  will  the  short  one  vibrate  while  the  long  one  is 
vibrating  four  times  ? 

5.  What  must  you  do  to  a  pendulum  to  make  it  vibrate  three 
times  as  fast? 

6.  A  clock  gains  time.    What  is  the  trouble  with  its  pendulum? 

7.  Two  pendulums  are  4  and  9  feet  long  respectively.    How 
many  vibrations  will  the  short  one  make  while  the  long  one  is 
vibrating  twice? 

8.  Two  pendulums  are  respectively  49  and  64  inches  long.    How 
will  their  times  of  vibration  compare? 

9.  How  long  must  a  pendulum   be  to  vibrate  once  in  tw« 
»econds  ? 

10.  How  long  must  a  pendulum  be  to  vibrate  twice  a  second  ? 


§  94  ENERGY.  57 

SECTION     V. 
ENERGY. 

92.  Work.  —  In  physical  science,  the  term  work 
signifies  the  overcoming  of  resistance  of  any  kind. 
Whether  this  overcoming  of  resistance  is  pleasant  or  not 
does  not  enter  into  consideration  here,  all  play  being  a 
species  of  work.     The  word  is  here  used  in  this  enlarged 
sense. 

93.  Energy.  —  Energy   is  the  power  of  doing 
work. 

If  one  man  can  do  more  work  than  another,  he  has 
more  energy.  If  a  horse  can  do  more  work,  in  a  given 
time,  than  a  man,  the  horse  has  more  energy  than  the 
man.  If  a  steam-engine  can  do  more  work  than  a  horse, 
it  has  more  energy.  If  a  moving  cannon-ball  can  over- 
come a  greater  resistance  than  a  base-ball,  it  has  more 
energy. 

94.  Elements   of  Work   Measure.  —  Imagine  a 
flight  of  stairs,  each  step  having  a  rise  of  twelve  inches. 
On  the  floor  at  the  foot  of  the  stairs  are  a  one  pound 
weight  and  a  ten  pound  weight.     Lift  the  first  weight 
to  the  top  of  the  first  step.    How  much  work  have  you 
performed  ?  .Perhaps  you  will  answer,  one  pound  cf  work. 
Now  place  the  second  weight  beside  tho  first.     How 
much  work  did  you  perform  in  so  doing :     Perhaps  you 
will  say  ten  times  as  much  as  before  or  ten  pounds. 


58  NATURAL   PHILOSOPHY.  §  94 

Now  lift  each  of  them  another  step,  and  then  another, 
until  they  rest  on  the  top  of  the  tenth  step.  To  lift  the 
heavier  weight  the  second,  third  and  following  times 
involved  as  much  work  each  time  as  to  lift  it  the  first 
foot,  but  you  would  hardly  say  that  you  had  lifted  a 
hundred  pounds. 

Still  it  is  sure  that  to  place  it  on  the  tenth  step 
required  just  ten  times  as  much  work  as  it  did  to  place 
it  on  the  first  step  or  just  one  hundred  times  as  much 
work  as  it  did  to  place  the  one  pound  weight  on  the 
first  step. 

It  is  evident  that  the  two  elements  of  weight 
and  height  are  necessarily  considered  in  meas- 
uring the  work  performed. 

95.  Units  of  Work ;  the  Foot-pound.  —  It  is 
often  necessary  to  represent  work  numerically  and  so  we 
need  a  unit  of  measurement.  The  unit  commonly  in 
use,  for  the  present,  in  England  and  this  country  is  the 
foot-pound. 

A  foot-pound  is  the  amount  of  work  required 
to  raise  one  pound  one  foot  high  against  the 
force  of  gravity. 

The  work  required  to  raise  one  kilogram  one  meter 
high  against  the  same  force  is  called  a  Mogram-meter. 

(a.)  To  get  a  numerical  estimate  of  work,  we  multiply  the  num- 
ber of  weight  units  raised  by  the  number  of  units  of  length  in  the 
vertical  height  through  which  the  body  is  raised.  A  weight  of  25 
pounds  raised  3  feet,  or  one  of  3  pounds  raised  25  feet,  represents 
75  foot-poun4f. 


§  98  ENERGY.  59 

96.  Rate  of  Doing  Work.  —  In  measuring  work 
done,  the  time  employed  is  not  taken  into  consideration. 
In  estimating  the  power  that  is  to  do  the  work,  the  time 
is  an  important  element.     Lifting  a  ton  100  feet  high 
involves  200,000  foot-pounds,  whether  the  work  be  done 
in  ten  minutes  or  ten  hours.     But  the  engine  that  can 
do  this  work  in  ten  minutes  is  more  powerful  than  one 
that  requires  ten  hours. 

97.  Horse-Power.  —  <A  horse-power  represents 
the  ability  to  perform  33,000  foot-pounds  in  a 
minute  or  550  foot-pounds  in  a  second. 

An  engine  that  can  do  66,000  foot-pounds  in  a  minute 
or  33,000  foot-pounds  in  half  a  minute  is  called  a  two 
horse-power  engine.  To  compute  the  number  of  horse- 
powers represented  by  an  engine  at  work,  multiply  the 
number  of  pounds  raised  by  the  number  of  feet,  and 
divide  the  product  by  33,000  times  the  number  of 
minutes  required  to  do  the  work. 

Experiment  40.— Into  a  pail  full  of  moist  clay  or  stiff  mortar, 
drop  a  bullet  from,  a  height  of  one  yard.  Notice  the  depth  to 
which  the  bullet  penetrates.  Drop  the  bullet  from  a  height  of 
four  yards.  It  will  strike  the  clay  with  twice  the  velocity  (§  79) 
and  penetrate  four  times  as  far  as  it  did  before. 

98.  Relation  of  Velocity  to  Energy. — If  the  last 
experiment  be  continued  by  dropping  the  bullet  from  a 
height  of  nine  yards,  it  would  strike  the  clay  with  a 
three-fold  velocity  and  penetrate  to  nine  times  the  depth. 
The  work  done  by  a  moving  body  will  vary  as  the  mass 
and  as  the  square  of  the  velocity. 


60  NATURAL   PHILOSOPHY.  §  98 

(a.)  The  energy  of  a  moving  body  may  be  computed  in  foot- 
pounds by  multiplying  the  number  of  pounds  in  the  moving  body 
by  the  square  of  the  number  of  feet  it  is  moving  per  second  and 
dividing  the  product  by  6432  (or  twice  the  increment  of  velocity 
due  to  gravity,  §  81). 

Kinetic  Energy  =  ^. 

NOTE.— For  a  fuller  discussion  of  this  subject,  see  the  author's 
Elements  of  Natural  Philosophy,  §  157. 

99.  Two  Types  of  Energy. — There  are  two  kinds 
of  energy.  One  is  called  energy  of  motion ;  the  other, 
energy  of  position. 

A  falling  weight  or  running  stream  possesses  energy 
of  motion  ;  it  is  able  to  overcome  resistance  by  reason  of 
its  weight  and  Telocity. 

But,  before  the  weight  began  to  fall,,  while  it  was  at 
rest,  it  had  the  power  of  doing  work  by  reason  of  its 
elevated  position  with  reference  to  the  earth.  When  the 
water  of  the  running  stream  was  at  rest  in  the  lake 
among  the  hills  it  had  a  power  of  doing  work,  an  energy,  . 
which  was  not  possessed  by  the  waters  of  the  pond  in 
the  valley  below.  This  energy  or  power  of  doing  work 
results  from  its  peculiar  position. 

The  weight  or  the  water,  when  thus  elevated  and  mo- 
tionless, has  no  working  power  in  the  sense  with  which 
we  are  most  familiar  with  it.  Yet  it  is  very  clear  that 
it  is  possible,  under  certain  conditions,  for  it  to  do  work. 
We  might,  therefore,  use  the  term  "possible  energy"  to 
denote  the  power  in  question  and,  as  a  matter  of  fact, 


§  100  ENERGY.  61 

the   term    "  potential  energy,"  which  means  the  same 
thing,  is  thus  used. 

Energy  of  motion  is  called  kinetic  energy; 
energy  of  position  is  called  potential  energy. 

ioo.  Convertibility  of  Kinetic  and  Potential 
Energies.  —  We  may  at  any  moment  convert  kinetic 
energy  into  potential,  or  potential  energy  into  kinetic. 
One  is  as  real  as  the  other  and,  when  it  exists  at  all, 
exists  at  the  expense  of  a  definite  amount  of  the  other. 

Imagine  a  ball  thrown  upward  with  a  velocity  of  64.32 
feet.  As  it  begins  to  rise  it  has  a  certain  amount  of 
kinetic  energy  because  it  has  weight  and  velocity.  At 
the  end  of  one  second  it  has  a  velocity  of  only  32.16  feet. 
Consequently,  its  kinetic  energy  has  diminished.  It  has 
jost  some  of  its  velocity  but  it  has  gained  a  better  posi- 
tion for  doing  work.  Having  risen  48.24  feet,  it  has 
gained  a  considerable  potential  energy.  All  of  this  po- 
tential energy  results  from  the  kinetic  energy  which  has 
disappeared,  or  from  the  work  that  has  been  done. 

At  the  end  of  another  second  the  ball  has  no  velocity  ; 
it  has  reached  the  turning-point  and  is  at  rest.  Conse- 
quently, it  has  no  kinetic  energy.  But  the  energy  with 
whicli  it  began  its  flight  has  not  been  destroyed ;  it 
has  been  stored  up  in  the  ball  at  a  height  of  64.32  feet 
as  potential  energy.  If  at  this  instant  the  ball  be  caught, 
all  of  the  energy  may  be  kept  in  store  as  potential 
energy. 

If  now  the  'ball  be  dropped,  it  begins  to  lose  its  poten- 
tial and  to  gain  kinetic  energy.  When  it  reaches  the 


NATURAL  PHILOSOPHY. 


§  ioo 


ground  at  the  end  of  two  seconds  it  has  no  potential 
energy,  but  just  as  much  of  the  kinetic  type  as  was 
given  to  it  when  it  began  to  rise. 

In  a  simple  way  this  illustrates  the  important  fact 
that  energy  may  be  changed  from  one  form  to 
another  without  any  change  in  its  quantity. 

101.  Energy  a  Constant  Quantity.— In  the  case 
of  the  ball  thrown  upward,  at  the  start,  at  the  finish  or 
at  any  intermediate  point  of  either  its  ascent  or  descent, 
the  sum  of  the  two  types  or  kinds  of  energy  is  the  same. 
It  may  be  all  kinetic,  all  potential,  or  partly  both.  In 
any  case,  the  sum  of  the  two  continually  varying 
energies  is  constant.  Just  as  a  man  may  have  a  hun- 
dred gold  dollars,  now  in  his  hand,  now  in  his  pocket,  now 
part  in  his  hand  and  the  remainder  in  his  pocket ;  chang- 
ing a  dollar  at  a  time  from 
hand  to  pocket  or  vice  versa, 
the  amount  of  money  in  his 
possession  remains  constant, 
viz.,  one  hundred  dollars. 


102.  Pendulum  Illus- 
tration.— The  pendulum  af- 
fords a  good  and  simple  illus- 
tration of  kinetic  and  poten- 
tial energy,  and  of  their  power 
of  changing,  one  to  another, 
without  loss.  When  the  pen- 
dulum hangs  at  rest  in  a  vertical  position,  as  P  a,  it  has 
no  energy  at  all. 


FIG.  19. 


§  103  ENERGY.  63 

Considered  as  a  mass  of  matter,  separated  from  the 
earth,  it  certainly  has  potential  energy ;  but  considered 
as  a  pendulum,  it  has  no  energy. 

If  the  pendulum  he  drawn  aside  to  b,  we  raise  it 
through  the  space  a  h ;  that  is,  we  do  work,  or  spend 
kinetic  energy  upon  it.  The  energy  thus  used  is  now 
stored  up  as  potential  energy,  ready  to  be  changed  back 
into  energy  of  the  kinetic  type,  whenever  we  let  it  drop. 

As  it  falls  the  distance  ha,  in  passing  from  b  to  a, 
this  change  is  going  on.  When  the  pendulum  reaches  a, 
its  energy  is  all  kinetic  and  just  equal  to  that  spent  in 
raising  it  from  a  to  6.  This  kinetic  energy  now  carries 
it  on  to  c,  lifting  it  again  through  the  space  all.  Its 
energy  is  again  all  potential  just  as  it  was  at  b. 

If  we  could  free  the  pendulum  from  the  resistances 
of  the  air  and  friction,  the  energy  first  given  to  it  would 
swing  to  and  fro  between  the  extremes  of  all  potential 
and  all  kinetic;  but  at  every  point  of  the  arc  traversed, 
the  total  energy  would  be  an  unvarying  quantity,  always 
equal  to  the  energy  first  used  in  swinging  it  from  a  to  b. 

103.  Indestructibility  of  Energy. — Were  it  not 
for  friction  and  the  resistance  of  the  air,  the  pendulum 
would  vibrate  forever;  its  energy  would  be  indestruc- 
tible. Energy  is  withdrawn  from  the  pendulum  to  over- 
come these  impediments,  but  the  energy  thus  withdrawn 
is  not  destroyed 

What  becomes  of  it  will  be  seen  when  we  come  to 
study  heat  and  other  forms  of  energy,  which  result  from 


NATURAL  PHILOSOPHY. 


§103 


the  motions  and  positions  of  the  molecules  of  matter. 
The  truth  is  that  energy  is  as  indestructible  as 
matter.  For  the  present,  we  must  admit  that  a  given 
amount  of  energy  may  disappear  and  escape  our  search, 
but  it  is  only  for  the  present.  We  shall  soon  learn  to 
recognize  the  fugitive  even  in  disguise. 

104.  Recapitulation.— To  be  amplified  by  the  pupi] 
for  review. 


ENERGY. 


DEFINITION. 


WORK. 


Wtigkt. 
Height. 

f  Foot-pound. 

UNITS \  Kilogram-Metev 

1  Horse-Power. 


RELATION  TO  VELOCITY, 


(  KINETIC.       ) 

TYPES \  \  Convertikiliiy. 

[  POTENTIAL.  ) 


INDESTRUCTIBILITY. 


§  104  EXERCISES.  66 


EXERCISES. 

1.  What  is  the  necessary  horse-power  of  an  engine  that  is 
required  to  raise  100,000  pounds  198  feet  high  in  one  hour? 

An*.  10  H.  P. 

2.  How  long  will  it  take  a  2-horse-power  engine  to  raise  10  tons 
50  feet  high  ?  Ans.  15  min.,  10  sec.,  nearly. 

3.  How  much  must  you  increase  the  velocity  of  a  moving  body 
to  quadruple  its  energy  ? 

4.  How  does  the  energy  which  a  moving  body  must  expend 
before  it  can  come  to  rest  compare  in  amount  with  the  energy  pre- 
viously required  to  put  the  body  in  motion  ? 

5.  Define  force  and  energy. 

6.  What  amount  of  work  is  needed  to  lift  20  bricks  weighing  5 
pounds  each  50  feet  high  ? 

7.  (a.)  How  much  work  could  the  bricks  mentioned  in  Exer- 
cise 6  perform  in  falling  back  to  the  ground  ?    (6.)  Where  did  they 
get  that  energy  ? 

8.  How  much  work  may  be  done  by  a  ball  weighing  64,32C 
pounda  and  striking  with  a  velocity  of  200  feet  a  second  ? 

Am.  40,000,000  foot-pounds. 


NATURAL  PHILOSOPHY.  §  104 


HE  VIEW     QUESTIONS. 

1.  If  a  cork  be  released  at  the  bottom  of  a  vessel  of  water,  it 
quickly  rises  to  the  surface.     Explain  the  upward  motion  of  the 
cork. 

2.  Give  some  illustration  of  unstable  equilibrium  not  mentioned 
in  this  book. 

3.  Why  does  a  wagon  with  a  ton  of  hay  overturn  more  easily 
than  a  similar  wagon  with  a  ton  of  stone  ? 

4.  In  loading  a  wagon,  where  should  the  heavy  articles  be 
placed?    Why? 

5.  Why  do  quadrupeds  leam  to  walk  more  easily  than  bipeds  ? 
G.  Imagine  a  man  suspended  in  otherwise  empty  space.     Could 

he  put  himself  in  motion  ?    Why? 

7.  If  the  distance  between  two  bodies  be  doubled,  how  will 
their  attraction  for  each  other  be  affected  ? 

8.  What  is  meant  by  "  increment  of  gravity  "  ? 

9.  Why  have  liquids  no  permanent  shape  ? 

10.  What  property  of  steel  fits  it  for  use  in  pens  ? 

11.  What  property  of  matter  is  illustrated  in  the  action  of  the 
part  of  a  watch  that  makes  the  wheels  move  ? 

12.  What  force  resists  our  attempts  to  draw  a  nail  out  of  wood  ? 

13.  What  is  the  object  of  ballast  in  a  vessel  ? 

14   Tli  rust  your  hand  into  water  and  it  comes  out  wet.     What 
property  of  matter  is  illustrated  by  the  experiment  ? 
15.  What  is  the  cause  of  weight? 


CHAPTEfi    Hi* 

SIMPLE     MACHINES. 


SECTION     I. 

PRINCIPLES   OF    MACHINERY.— THE    LEVER. 

Experiment  41.— Arrange  two  small  pulleys  and  a  spring-bal 
ance  as  shown  in  the  figure.  The  pulleys  and  balance  may  bt 
bought  at  the  hardware  store  and  will  be 
much  used.  If  W  weighs  10  pounds,  the 
spring-balance  at  P  will  show  that  the  hand 
exerts  a  pull  of  5  pounds.  If  the  hand  moves 
two  feet  it  will  be  noticed  that  W  moves  one 
foot.  No  matter  how  far  P  moves ;  W  will 
move  just  half  as  far  in  the  same  time.  Under 
all  circumstances,  the  product  of  the  number 
representing  the  weight  of  W  into  the  number 
representing  the  distance  it  moves,  will  equal 
the  product  of  the  number  representing  the 
weight  (or  pull)  at  P  into  the  number  repre- 
senting the  distance  it  moves.  Change  the 
positions  of  P  and  W  and  see  if  this  is  so. 

FIG.  19. 

NOTE. — In  this  case,  if  the  apparatus  be 

delicate,  it  will  be  necessary  to  make  an  allowance  for  the  weight 
of  the  cord  and  movable  pulley.  This  allowance  may  be  made 
by  hanging  just  enough  weight  at  P  to  keep  the  apparatus  in 


68  NATURAL  PHILOSOPHY.  §  105 

equilibrium  before  W  is  attached.  In  all  experiments  with 
pulleys,  levers,  and  other  machines,  it  is  proper  to  see  that  the 
machine  itself  is  in  equilibrium  before  attempting  to  determine  the 
relations  of  power  to  weight. 

105.  What   is   a   Machine  ? — A  machine  is  a 
contrivance   by   means   of  which   a  given  power 
may  be  used  to  overcome  a  given  resistance  with 
certain  advantages. 

Its  use  is  to  transform  the  intensity  of  energy,  so  that 
an  energy  of  small  intensity,  acting  through  a  consider- 
able distance,  may  be  made  to  do  the  same  work  as  a 
]arge  power,  acting  through  a  small  distance,  or  rice 
versa. 

106.  A  Machine  can  not  Create  Energy. — No 
machine  can  create  or  increase  energy.     In  fact,  the  use 
of  a  machine  causes  a  waste  of  power,  for  a  part 'of  the 
energy  must  be  used  to  overcome  the  friction  of  the  ma- 
chine itself,  thus  diminishing  the  amount  that  can  be 
used  for  doing  useful  work. 

107.  Of  what  Use  are  Machines  ?— Some  of  the 
many  advantages  resulting  from  the  use  of  machines  are : 

(1.)  They  enable  us  to  do  work  more  quickly 
than  we  otherwise  could,  as  in  the  sewing- 
machine  or  spinning-wheel. 

(2.)  They  enable  us  to  do  work  that  we  other- 
wise could  not  do  at  all,  as  in  lifting  a  large 
stone  with  a  crow-bar  or  pulleys. 


§  108  PRINCIPLES   OF  MACHINERY.  09 

(3.)  They  enable  us  to  change  the  direction  of 
our  force,  as  in  hoisting  a  flag  on  a  flag-staff. 
It  would  be  inconvenient  to  climb  the  pole  and 
then  draw  up  the  flag. 

(4.)  They  enable  us  to  employ  other  forces  than 
our  own,  as  the  strength  of  animals,  the  forces 
of  wind,  water,  steam,  etc. 

108.  General  Laws  of  Machines.— The  work  to 
be  done  by  a  machine  is  generally  called  the  weight  or 
load.  The  work  of  the  power  (e.  </.,  foot-pounds)  is 
always  equal  to  the  work  of  the  load,  the  power  expended 
in  the  machine  itself  being  disregarded.  The  following 
are  called  the  general  laws  of  machines : 

(1.)  What  is  gained  in  intensity  of  power  is 
lost  in  time,  velocity,  or  distance;  and 
what  is  gained  in  time,  velocity,  or  distance,  is 
lost  in  intensity  of  power. 

(2.)  The  power  multiplied  by  the  distance 
through  which  it  moves,  equals  the  weight 
multiplied  by  the  distance  through  which 
it  moves.  For  example,  if  the  power  moves 
five  times  as  far  as  the  weight,  the  power  will 
be  only  one-fifth  as  great  as  the  weight. 

(3.)  The  power  multiplied  by  its  velocity,  equals 
the  weight  multiplied  by  its  velocity. 
For  example,  if  the  power  moves  five  times  as 
fast  as  the  weight,  the  power  will  be  only  one- 
fifth  as  great  as  the  weight, 


70 


NATURAL   PHILOSOPHY. 


§109 


109.  What  is  a  Lever  ? — A  lever  is  an  inflex- 
ible bar   capable  of  being  freely  moved  about  a 
fixed  point  or  line,  called  the  fulcrum. 

Every  lever  has  two  arms.  The  power-arm  is  the 
perpendicular  distance  from  the  fulcrum  to  the  line  in 
which  the  power  acts;  the  weight-arm  is  the  perpen- 
dicular distance  from  the  fulcrum  to.  the  line  in  whioh 
the  weight  acts. 

1 10.  Classes   of  Levers. — There  are  three  classes 
of  levers,  depending  upon  the  relative  positions  of  the 
power,  weight,  and  fulcrum. 

(1.)  If  the  fulcrum  is  between  the  power  and 
weight  (P.  F.  W.),  the  lever  is  of  the  first  class  (Fig.  20); 
fl  F  e.  g.,  crow-bar,  balance, 

steel-yard,  scissors,  pincers. 

(2.)  If  the  weight  is 
between  the  power  and 
the  fulcrum  (P.  W.  F.),  the 
lever  is  of  the  second 
class,  (Fig.  21) ;  e.  g.,  cork- 
squeezer,  nut-cracker,  wheel- 
barrow. 


FIG.  30. 


FIG.  21. 


(  3.)  If  the  power  is  be- 
tween the  weight  and  the 
fulcrum  (W.  P.  F.),  the 
lever  is  of  the  third  class 
(Fig.  2?) ;  e.  g.,  fire-tongs, 
sheep-shears,  human  fore- 
arm. 


§  III  THE   LEVER.  71 

Experiment  42. — Bore  a  small  hole  18  inches  from  one  end  of 
a  yard-stick  and  nearer  one  edge  than  the  other.  Thrust  a  round 
metal  pin  through  this  hole  into  a  firm  vertical  support,  which 
may  be  made  by  nailing  an  inch  board,  6  inches  by  4  feet,  to  the 
side  of  a  soap  box  and  placing  bricks  in  the  box.  Such  a  support 
will  be  convenient  for  many  purposes.  The  pin  should  be  of 
such  a  size  as  to  move  easily  in  the  hole  and  yet  have  strength 
enough  to  carry  a  load  of  several  pounds.  It  will  be  well  to  put  a 
email  metal  washer  on  the  pin  between  the  yard  stick  and  the 
support.  Be,  sure  that  tlie  stick  is  in  equipoise  (or  balances  on  the 
pin),  shaving  off  the  upper  edge  of  the  stick  near  one  end  if  neces- 
sary for  this  purpose.  Borrow,  buy  or  (best  of  all)  make  two  sets 
of  weights  of  \  lb.,  1  Ib.  and  2  Ib.  respectively.  The  weights 
may  be  provided  with  suspension  loops  of  linen  or  silk  thread, 
the  weight  of  which  may  be  disregarded. 

(d.)  Hang  an  8  oz.  weight  on  each  side  of  the  pin,  6  inches 
distant.  Notice  that  this  lever  of  the  first  class  balances; 
then  remove  the  weights. 

(6.)  Hang  a  1  lb.  weight  on  each  side  of  the  pin,  12  inches 
distant.  The  lever  balances.  Remove  the  weights. 

(c.)  Hang  a  \  lb.  weight  on  one  side  of  the  pin  16  inches 
distant  and  a  1  lb.  weight  on  the  other  side  of  the  pin,  8  inches 
distant.  The  lever  balances.  Remove  the  weights. 

(d.)  Hang  a  £  lb.  weight  on  one  side  of  the  pin  16  inches 
distant  and  a  2  lb.  weight  on  the  other  side  of  the  pin,  4  inches 
distant.  The  lever  balances. 

in.  Static  Laws  of  the  Lever.  —  It  will  be 
clearly  seen  from  the  last  experiment,  that  the  following 
statements  are  true : 

(1.)  The  power  multiplied  by  the  power -arm 
equals  the  weight  multiplied  by  the 
weight-arm;  or, 


72  NATURAL  PHILOSOPHY.  §  1 12 

(2.)  A  given  power  will  support  a  weight  as 
many  times  as  great  as  itself,  as  the 
power -arm  is  times  as  long  as  the 
weight-arm. 

NOTE. — A  static  law  expresses  the  relation  between  the  power 
and  weight  when  the  machine  is  in  equilibrium.  In  order  that 
there  be  motion,  one  of  the  products  mentioned  in  the  law  above 
must  be  greater  than  the  other.  The  lever  itself  must  be  in  equi- 
librium before  the  power  and  weight  are  applied.  It  is  to  be 
noticed  that  when  we  speak  of  the  power  multiplied  by  the  power- 
arm,  we  refer  to  the  abstract  numbers  representing  the  power  and 
power-arm.  We  can  not  multiply  pounds  by  feet,  but  we  can 
multiply  the  number  of  pounds  by  the  number  of  feet. 

Experiment  43.— Make  two  scale-pans  by  hanging  two  tin  can 
covers  (each  by  three  or  four  stout  threads  about  a  foot  Icng)  from 
the  balanced  yard  stick  of  Experiment  42.  The  upper  ends  of 
the  cords  of  each  pan  may  be  knotted  together  and  provided  with 
a  loop  that  will  easily  slide  over  the  end  of  the  yard-stick.  Place 
one  of  these  loops  an  inch  from  each  end  of  the  yard-stick  and  be 
sure  that  your  scales  balance  well.  Place  an  8  oz.  weight  in  one 
scale  pan  and  weigh  out  ^  Ib.  of  sand. 

112.  The  Balance. — The  balance  is  essentially 
a  lever  of  the  first  class,  having  equal  arms. 

Its  use  is  to  determine  the  weights  of  bodies.  The 
lever  itself  is  called  the  beam.  The  ends  of  the  beam 
carry  two  pans,  one  to  support  the  weights  used,  the 
other  to  support  the  article  to  be  weighed.  (Fig.  23.) 

(a.)  That  the  balance  may  be  accurate,  the  arms  must  be  of  the 
same  length.  To  make  these  arms  exactly  equal  is  far  from  an 
easy  task.  Balances  are  made  so  delicate  that  they  may  be  turned 
by  less  than  a  thousandth  of  a  grain.  A  really  good  balance  is  an 
expensive  piece  of  apparatus  and  requires  great  care. 


THE  LEVER. 


FIG.  23. 


113.  False  Balances.  —  False  balances  (levers 
of  the  first  kind  with  unequal  arms)  are  some- 
times used  by  dishonest  dealers. 

When  buying,  they  place  the  goods  on  the  shorter 
arm ;  when  selling  on  the  longer.  The  cheat  may  be 
exposed  by  changing  the  goods  and  weights  to  the  oppo- 
site sides  of  the  balance.  The  true  weight  may  be  found 
by  weighing  the  article  first  on  one  side  and  then  on  the 
other,  and  taking  the  geometrical  mean  of  the  two  false 
weights  ;  that  is,  by  finding  the  square-root  of  the  pro- 
duct of  the  two  false  weights. 

For  instance,  if  the  article  appears  to  weigh  eight 
pounds  on  one  side  and  six  pounds  and  two  ounces  on 
the  other,  the  true  weight  is  seven  pounds. 

8  x  6^  =  49.      VT9~=  7, 


74  NATURAL  PHILOSOPHY.  §  114 

In  buying  seven  pounds  of  butter,  the  dealer  would 
pay  for  only  six  pounds  and  two  ounces  and,  in  selling 
the  same  butter,  he  would  get  pay  for  eight  pounds, 
thus  gaining,  by  fraud,  the  price  of  one  pound  and  four- 
teen ounces,  in  addition  to  his  legitimate  profit. 

114.  Double  Weighing.  —  The  true  weight  of  a 
body  may  be  found  with  a  false  balance  in  another  way. 
The  article  to  be  weighed  is  placed  in  one  pan,  and  a 
counter-weight,  as  of  shot  or  sand,  placed  in  the  other 
pan  until  equilibrium  is  produced.     The  article  is  then 
removed  and  known  weights  placed  in  the  pan  until 
equilibrium  is  again  produced.     These  known  weights 
will  be  the  true  weight  of  the  given  article. 

115.  Load    between    Two    Supports.  —  If   a 
beam    rest    on    two   supports    and    carry   a    load 
between    them,  the  beam  may   be    considered    a 
lever  of  the  second  class. 

The  part  carried  by  either  support  may  be  found  by 
considering  it  as  the  power  and  the  other  support  as  the 
fulcrum.  (Fig.  24.)  For  instance,  if  the  string  of  fish 
weighs  15  pounds  and  is  placed  two  feet  from  the  boy  in 
front  and  eight  feet  from  the  boy  in  the  rear,  we  may 
want  to  know  how  much  of  the  load  thi  latter  is  carry- 
ing. 

In  this  case,  the  shoulder  of  the  boy  in  front  is 
called  the  fulcrum.  The  weight- arm  is  two  feet  and 
the  power-arm  is  ten  feet.  As  the  power-arm  is  five 
tiroes  as  long  as  tb.e  weight-arm,  th§  weight  will  be  five 


§"6 


THE  LEVER. 


75 


times  the  power  or  the  power  will  be  one-fifth  the  weight, 
viz.,  three  pounds.  Of  course,  the  boy  in  front  carries 
the  other  twelve  pounds  of  the  load. 


FIG.  24. 

If  W8  prefer,  we  may  call  the  shoulder  of  the  boy  in 
the  rear  the  fulcrum.  In  this  case,  the  weight-arm  is 
eight  feet  and  the  power-arm  is  ten  feet.  As  the  weight- 
arm  is  four-fifths  as  long  as  the  power-arm,  the  power 
will  be  four-fifths  of  the  weight,  viz.,  twelve  pounds. 
Of  course,  the  other  three  pounds  is  borne  by  the  ful- 
crum, or  the  boy  in  the  rear. 


116.  Compound  Lever. — Sometimes  it  is  not  con- 
venient to  use  a  lever  sufficiently  long  to  make  a  given 
power  support  a  given  weight.  A  combination  of  levers 
palled  a  compound  lever  may  then  be  used. 


76 


NATURAL   PHILOSOPHY. 


§n<5 


may  be  mentioned  as  a  familiar  illustration  of  the  com- 
pound lever.  In  this  case  we 
have  the  following: 

Statical    Law.  — The   con- 
tinued  product  of  the  power 
and  the  lengths  of  the  alter- 
nate  arms,  beginning    with 
the    power -arm,   equals   the 
continued     product    of    the 
weight   and    the   lengths   of 
the    alternate    arms    begin- 
ning with  the  weight-arm, 
(a.)  If  the  arms  of  the  lever,  a,  (Fig.  25)  be  1^  feet  and  6  feet  ; 
of  lever  b,  1  foot  and  3  feet ;  of  lever  c,  2  feet  and  5  feet,  a  pull  of 
7  pounds  at  P  will  support  a  weight  of  210  pounds  at  W. 
7x6x3x5  =  210  x2xlxU. 


FIG.  25. 


117.  Recapitulation, 
for  review. 


-To  be  amplified  by  the  pupil 


MACHINES. 


DEFINITION. 

NO  CREATIVE  POWER. 

USES. 

LAWS. 


LEVER... 


DEFINITION. 


CLASSES 


LAW. 
COMPOUND. 


...  J  False. 


FIRST:  Balance 

[  Double  Weighing. 
SECOND  :  Load  between  Two  Supports 

THIRD. 


EXERCISES.  ?? 


EXERCISES. 

1.  The  power-arm  of  a  lever  must  be  how  many  times  as  long 
as  the  weight-arm  to  have  100  kilograms  support  1,000  kilograms'! 

2.  Why  is  the  short  arm  of  a  steelyard  larger  around  than  the 
long  arm? 

3.  In  Fig.  20,  if  the  weight-arm  is  one  foot  and  the  lever  is  five 
feet  long,  what  weight  at  W  will  be  supported  by  a  pull  of  seven 
pounds  at  P  ? 

4.  In  Fig.  21,  if  the  weight-arm  is  one  foot  and  the  lever  is  five 
feet  in  length,  what  weight  at  W  will  be  supported  by  a  pull  of 
seven  pounds  at  P  ? 

5.  In  Fig.  22,  if  the  power-arm  is  one  foot  and  the  length  of  the 
Jever  is  five  feet,  what  pull  at  P  will  be  needed  to  support  seven 
pounds  at  Wt  Ans.  35  Ib. 

6.  A  lever  is  75  inches  long.     It  enables  a  weight  of  one  pound 
to  balance  a  load  of  two  pounds.     Is  the  fulcrum  in  the  middle  1 
Why  ?     If  not,  how  far  must  the  fulcrum  be  from  the  end  of  the 
lever? 

7.  The  weight-arm  of  a  lever  is  6  feet  long  ;  the  power-arm  is 
12  feet  long,     (a.)  What  is  the  length  of  the  lever  if  it  be  of  the 
first  class?    (6.)  If  of  the  second  class?    (c.)  If  of  the  third  class? 

8.  Where  must  a  straight  bar  be  supported  so  that  a  pound 
weight  hung  at  one  end  will  support  two  pounds  at  the  other  endl 

Ans.  At  |  its  length  from  one  end. 

9.  The  toggle  joint   shown   in   Fig.   26  and 
similar  in   action   to   those   commonly   used   on 
carriage  tops  is  used  for  punching  iron.     While 
the  joint  is  moved  from  c  to  d,  the  punch  b  is 
moved  forward  until  the  distance  ab  becomes  ac 
+  bc.      If  e d  —  10  inches  ;   nb,  99 1  inches;  ac 
and  be  each,    50  inches  and   a   power  of  1,000 
pounds  pull  c  toward  d,  what  force  will  be  ex- 
erted upon  the  punch  at  6  ? 

Ant.  20,000  Ib. 

FIG.  26. 


?S  XATTJRAL  PHILOSOPHY.  §  Il8 

SECTION     II. 

THE  WHEEL  AND   AXLE   AND   THE    PULLEY. 

118.  The  Wheel   and   Axle.  —  The  wheel  and 
axle  consists  of  a  wheel  united  to  a  cylinder  in 
such   a   way  that   they  revolve   together  about  a 
common  axis. 

The  power  being  applied  to  the  circumference  of  the 
wheel,  the  load  is  carried  by  a  rope  wound  around  the 
axle. 

119.  Advantages  of  the  Wheel   and  Axle.— 

The  ordinary  crowbar  or  other  lever  of  the  first  class 
can  lift  a  load  only  a  short  distance 
at  one  time.  In  order  to  raise  the 
load  higher  tban  the  vertical  dis- 
tance through  which  the  weight 
end  of  the  lever  passes,  it  is  neces- 
sary to  support  the  load  and  re- 
adjust the  fulcrum.  The  motion 
is  irregular  and  time  is  lost.  These 

difficulties  are  obviated  by  using  the  wheel  and  axle. 

120.  Law  of  the  Wheel  and  Axle.  —The  power 
multiplied   by   the   radius,  diameter   or  circum- 
ference of  the   wheel   equals   the    weight   multi- 
plied by  the  corresponding  dimension  of  the  axle, 
or, 

The    power   will    support    a    weight    as    many 


TfrHEEL  AtfD  AXLE. 


times  as  t'reat  as  itself,  as  the  radius,  diameter 
or  circumference  of  the  wheel  is  times  as  great 
as  the  siivilar  dimension  of  the  axle. 

Example.  -  If  the  radius,  diameter  or  circumference  of  the 
wheel  is  t<  •  times  as 
great  as  th»  similar  di- 
mension of  ae  axle,  the 
power  will  move  ten 
times  as  far  and  ten 
times  as  fast  as  the 
weight  and  the  weight 
will  be  ten  times  as 
great  as  the  power.  A 
power  of  15  pounds  will 
support  a  load  of  150  FIG.  28. 

pounds.     See  §  108,  (2)  and  (3). 

121.   Various   Forms   of  V/heel   and   Axle.— 

The  wheel  and  axle  appears  in  various  forms.     It  is  not 
necessary  that  an  entire  wheel  be  present,  a  single  spoke 
or  radius  being  sufficient  for  the  application  of  the  power, 
as  in  the  case  of  the  windlass  (Fig. 
28)  or  capstan  (Fig.  29).      In  all 
such  cases,  the  radius  being  given, 
the  diameter  or  circumference  of 
the  wheel  may  be  easily  computed. 
(See  Appendix  A.} 

In  one  of  the  most  common 
forms,  the  power  is  applied  by 
means  of  a  rope  wound  around  the  circumference  of  the 
wheel.  When  this  rope  is  unwound  by  the  action  of  the 
power,  another  rope  is  wound  up  by  the  axlo  and  the 
weight  thus  raised. 


FIG.  29. 


so 


NAfUttAL  PHILOSOPHY. 


§122 


122.  Wheel-work. — Another  method  of  securing  a 
great  differeno  in  the  in- 
tensities of  bala  iiced  forces, 
is  to  use  a  con  bination  of 
wheels  and  axles  i  f  moderate 
size.  Such  a  combina- 
tion constitutes  a  train. 
The  wheel  that  imparts  the 

A  motion  is  called  the  driver  ; 

<  |  that  which  receives  it,  the 
follower.  An  axle  with 
teeth  upon  it  is  called  a 
pinion.  The  teeth  or  cogs 


FIG.  30. 


of  a  pinion  are  called  leaves. 

123.  Ways    of   Connecting    Wheels.  —  Wheels 
may  be  connected  in  three  ways  : 

(1.)  By  the  friction  of  their  circumferences. 
(2.)  By  lands  or  belts. 
(3.)  By  teeth  or  cogs. 

The  third  of  these  methods  has  been  already  con- 
sidered. 

124.  Uses  of  the  First  Two  Ways.  — The  first 
method  is  used  where  no  great  resistance  is  to  be  over- 
come but  where  evenness  of  motion  and  freedom  from 
noise  are  chiefly  desired.      It  is  illustrated   in  some 
sewing-machines. 

The  second  method  is  used  when  the  follower  is  to  be 
at  some  distance  from  the  driver.     The  friction  of  the 


§127 


THE  PULLEY. 


81 


belt  upon  the  wheels  must  be  greater  than  the  resistance 
to  be  overcome.  It  is  il  lustrated  in  most  sewing-machines, 
in  the  spinning-wheel  and,  on  a  large  scale,  in  every 
machine  shop. 

125.  Relative  Velocities  Determined.— Tlie  fol- 
lower ivill   revolve  as  main/  ihucs  as  fast  as  tlie 
driver,  as  its  radius,  diameter  or  circumference 
is  contained  times  in  that  of  the  driver. 

(a.)  If  the  driver  have  a  circumference  of  10  feet  and  the  fol- 
lower a  circumference  of  2  feet,  the  follower  will  revolve  5  times 
as  fast  as  the  driver. 

126.  What  is  a  Pulley  ?— A  pulley  consists  of  a 
wheel  turning  upon  an  axis   and  having  a  cord 
passing  over  its  grooved,  circumference. 

The  frame  supporting  the  axis  of  the  wheel  is  called 
the  block. 


127.  A  Fixed  Pulley. — If  a  cord 
be  passed  over  a  pulley  fixed  to  the 
ceiling,  a  weight  being  at  one  end  and 
the  hand  applied  at  the  other,  as  at 
P  in  Fig.  31,  the  hand  will  have  to 
exert  a  force  equal  to  the  weight  of 
the  load.  If  the  weight  be  moved, 
the  hand  and  weight  will  move  equal 
distances.  It  is  evident,  then,  that 
the  fixed  pulley  affords  no  in- 
crease of  power,  but  only  change 
of  direction. 


FIG.  31. 


IfATtTRAL  PHILOSOPHY. 


128.  A  Movable  Pulley. — If  one  end  of  the  cord 
be  fastened  to  the  ceiling,  the  load  suspended  from  the 

pulley,  and  the  other  end  of  the  cord 
drawn  up  by  the  hand  or  passed  over 
a  fixed  pulley,  as  shown  in  Fig.  32,  it 
will  be  evident  that  the  fixed  support 
(the  hook)  carries  half  the  load  and 
the  hand  the  other  half. 

To  raise  the  weight  one  foot,  the 
hand  must  pull  up  two  feet  of  the 
cord ;  that  is  to  say,  each  section  of 
the  cord  carrying  the  weight  must  be 
shortened  one  foot.  Thus  the  hand, 
by  pulling  50  pounds  two  feet,  is  able 
to  raise  100  pounds  one  foot. 

129.  Advantages   of  the   Pulley.  —  It  is  to  be 
very  carefully  noticed  that  the  pulley  does  not  create  any 
energy  or  do  any  work.    It  simply  enables  us  to  exchange 
velocity  for  intensity  of  work,  and  to  change  the  direc- 
tion in  which  the  power  acts. 

If  the  hand  at  P  pulls  down  with  a  force  of  50  pounds 
and  moves  through  a  distance  of  four  feet,  it  performs 
200  foot-pounds  of  work.  At  the  same  time,  the  weight 
of  100  pounds  will  be  raised  two  feet  and  this  work  is 
also  represented  by  200  foot-pounds. 

It  also  requires  some  additional  poiver  to  overcome  the 
friction  of  the  machine  and  to  bend  the  ropes,  but  we 
probably  would  be  glad  to  pay  this  price  for  the  ability 
to  exchange  velocity  which  we  can  produce  for  the  in- 
tensity which  we  desire. 


§130 


THE  PULLET. 


130.    A  Combination  of  Pulleys.  —  By  the  use  ot 

several  fixed  and  movable  pulleys  in 
blocks,  the  number  of  parts  of  the 
cord  supporting  the  movable  block 
may  be  increased  at  pleasure. 

In  all  such  cases,  the  part  of 
the  cord  to  which  the  -power  is 
applied,  will  carry  only  a  part 
of  the  load.  The  value  of  this 
part  of  the  load  depends  upon  the 
number  of  sections  into  which  the 
movable  pulley  divides  the  cord. 

In  Fig.  33,  the  movable  pulley 
is  represented  as  dividing  the  cord 
into  six  such  parts.  Then  the  power 
applied  at  P  will  support  a  load  six 
times  as  great  as  itself. 

In  practice,  the  several  wheels  of 

each  block  are  made  of  the  pIG 

Wsame  size  and  placed  side 
by  side,  turning  upon  the  same  axis.     The  sev- 
eral wheels  in  a  block  are  often  called  sheaves. 


131.  Law    of  the    Pulley.  —  With   a 
puller/    having    a    continuous     cor  a,    a 
given   power   will  support   a    weight    as 
many  times  as  great  as  itself  as   there 
are    parts    of   the    cord    supporting    the 
movable  block. 

132.  Concerning:  the  Number  of  Parts 
FIG.  34.     of  the  Cord.  —  Look  at  the  several  figures  of 


84 


NATURAL  PHILOSOPHY. 


§132 


pulleys.  You  will  see  that  when  the  fixed  end  of  the 
cord  is  attached  to  the  fixed  block,  the  number  of  parts 
of  the  cord  supporting  the  weight  is  twice  the  number 
of  movable  pulleys  used. 

When  the  fixed  end  of  the  cord  is  attached  to  the 
movable  block,  the  number  of  parts  of  the  cord  is  one 
more  than  twice  the  number  of  movable  pulleys  used. 

133.  Recapitulation.— To  be  amplified  by  the  pupil 
for  review. 


WHEEL 
AND    AXLE. 


DEFINITIONS. 

ADVANTAGES. 

RELATION  TO  THE  LEVER. 

LAWS. 

FORMS. 

DRIVER. 

FOLLOWER. 


WHEEL  WORK... 


CONNECTIONS. 
RELATION  OF  P  TO  W. 


I  Modes. 
\  Uses. 


PULLEY... 


DEFINITION. 

1  FIXED. 
MOVABLE. 
COMBINATIONS. 
LAW. 

NUMBER  OF  PARTS  OF  THE  CORD. 


§  132  EXERCISES.  85 


EXERCISES. 

1.  If  the  wheel  (Fig.  27}  be  5  feet  in  diameter  and  the  axle  be 
1  foot,  what  power  must  be  exerted  by  the  hand  at  P  to  support  a 
load  of  125  pounds  at  IF?  An*.  25  Ib. 

2.  If  the  handle  of  a  windlass  (Fig.  28)  describes  a  circle  9  feet 
in  circumference  and  thus  causes  the  axle  to  wind  up  3  feet  of 
rope,  what  weight  at  W  will  be  supported  by  every  pound  of  force 
applied  at  P  ? 

8.  A  ship's  anchor,  weighing  two  tons,  is  to  be  hoisted  by  a 
cable  wound  around  the  barrel  of  the  capstan  (Fig.  29).  The 
barrel  is  two  feet  in  diameter,  A  man  pushes  at  the  end  of  each 
of  the  four  capstan  bars  (radii)  which  are  eight  feet  long.  With 
what  force  must  each  man  push  ?  Ana.  125  Ib. 

4  (a.)  With  such  a  pulley  as  is  represented  in  Fig.  31,  how 
great  a  load  will  a  pull  of  10  pounds  support  ?  (6.)  How  much 
with  such  a  pulley  as  is  represented  in  Fig.  32  ?  (e.)  How  much 
with  such  a  pulley  as  is  represented  in  Fig.  33  ?  (d.)  How  much 
with  such  a  pulley  as  is  represented  in  Fig.  34? 

5.  With  such  a  pulley  as  is  represented  in  Fig.  32,  how  great  a 
load  can  a  100-pound  boy  raise  ? 

Ana.  A  little  less  than  200  Ib. 

6.  A  man  who  weighs  180  pounds  lifts  a  weight  of  100  pounds 
by  means  of  a  fixed  pulley  over  his  head.     What  is  the  man's 
pressure  on  the  floor  ?  Ana.  80  Ib. 

7.  A  weight  of  4  pounds  is  hung  from  the  wheel  which  is  5  feet 
in  diameter.     A  weight  of  21  pounds  is  hung  from  the  axle  which 
is  1  foot  in  diameter.      Which  will  descend,  assuming  that  tb« 
wheel  and  axle  works  without  friction,? 


86 


NATURAL  PHILOSOPHY. 


§133 


SECTION     III. 

THE    INCLINED    PLANE,    WEDGE,    SCREW,    ETC. 


133- 


What  is  an  Inclined  Plane  ?  —  The  in- 
clined plane  is  a  sur- 
face sloping  so  as  to 
make  an  oblique  angle 
ivith  the  direction  of 
the  force  to  be  over- 
come. 


FIG.  35. 
against  the  force  of  gravity. 


In  most  cases,  it  is  used 
to  aid   in   lifting   bodies 


134.  Law  for  the  Inclined  Plane.  —  In  Fig.  36, 
the  plane  is  twice  as  long  as  it 
is  high.  As  there  indicated,  a 
force  of  ten  kilograms  will  sup- 
port a  weight  twice  as  heavy.  This 
result  may  be  easily  verified  by  ex- 
periment. We  may  establish  the 
following  law : 

Wlien  a  given  power  acts 
parallel  to  the  inclined  plane,  it  will  support  a 
u'eight  as  many  times  as  great  as  itself  as  the; 
of  the  plane  is  times  as  great  as  Us  ver* 


10  Kg.  ® 


FIG.  36. 


§138 


THE    WEDGE. 


87 


135-  What  is  a  Wedge  ? — A  wedge  is  a  mov- 
able inclined  plane. 

136.  Its  Use.  —  The 
wedge  is  used  for  mov- 
ing great  weights  short 
distances. 

A  common    method    of 

moving  bodies  is  to  place 

two  similar  wedges,   with  FlG-  37- 

their  thin  ends  overlapping,  under 
the  load.  Blows  of  equal  force 
are  struck  upon  the  heads  of  the 
wedges  at  the  same  time  and  the 
load  is  thus  raised  as  shown  by  the 
arrows  in  Fig.  38. 


FIG.  38. 


137.  A  More  Common  Use. — A  more  common 
kind  of  wedge  consists  of  two  inclined  'planes 
united  at  their  bases. 


Such  wedges  are  used  in  splitting  timber, 
stone,  etc.  The  power  is  given  in  repeated 
blows  instead  of  continued  pressure.  No 
definite  law  of  any  practical  value  can  be 
given  for  the  wedge,  further  than  that,  with 
a  given  thickness,  the  longer  the  wedge  the 
greater  the  gain  in  intensity  of  power. 


FIG.  39. 


138.  What  is  a  Screw  ? — A  screw  is 
a  cylinder,  generally  of  wood  or  nwfal,  with 


NATURAL   PHILOSOPHY. 


§138 


spiral   groove    or   ridge    winding    about    its    cir* 
cumference. 

The  spiral  ridge  is  called  the  thread  of  the  screw. 
The  thread  works  in  a  nut,  within  which  there  is  a 
corresponding  spiral  groove  to  receive  the  thread. 

(a.)  The  power  is  used  to 
turn  the  screw  within  a  fixed 
nut,  N,  or  to  turn  the  nut 
about  a  fixed  screw.  In  either 
case,  a  lever  or  wheel  is  gen- 
erally used  to  aid  the  power. 
Every  turn  of  the  screw  or  nut 
either  pushes  forward  the 
screw  or  draws  back  the  nut 
by  exactly  the  distance  be- 
tween  two  turns  of  the  thread, 
this  distance  being  measured, 
in  the  direction  of  F  W,  the 
axis  of  the  screw.  The  weight 

or  resistance  at  W  is  moved  this  distance,  while  the  power  at  P 
moves  over  the  circumference  of  a  circle  whose  radius  is  P  F. 


FIG.  40. 


(6.)  The  circumference  of  the  circle,  or  the  distance  that  the 
power  moves  while   the  weight  is  being    moved    the  distance 
between  two  turns  of  the  same  thread  of  the 
screw,  may  be  found  by  multiplying  two  times 
PF  (the  diameter  of  the  circle)  by  3.1416. 
See    Appendix   A.      The    difference    between 
these  two  distances  is  generally  very  great. 
Hence,  this  machine  affords  great  intensity  of 
power  with  a  corresponding  loss  of  velocity. 


(c.)  Figure  41  shows  that  the  screw  is  only 
a  modified  inclined  plane. 


FIG. 


§  ;<j.i  THE  SCREW.  89 

139.  Law  of  the  Screw.  —  With  the  screw,  a 
given  power  will  support  a  weight  as  many 
times  as  great  as  itself  as  the  circumference 
travelled  over  by  the  power  is  times  as  great  as 
the  distance  between  two  adjoining  turns  of  the 
thread. 


140.  Compound   Machines.  —  We  have  now  con- 
sidered each  of  the  six  simple  machines.     One  of  these 
may  be    made  to  act  upon  another  of  the  same  kind, 
as  in  the  case  of  the   compound  lever  or  wheel-work  ; 
or  upon  another  of  a  different  kind,  as  in  the  case  of 
the  endless  sciew. 

When  any  two  or  more  of  tfiese  machines  are  com- 
bined, the  effective  force  may  be  found  by  computing 
the  effect  of  each  separately  and  then  compounding 
them;  or  by  finding  the  weight  that  the  given  power 
will  support,  using  the  first  machine  alone,  considering 
the  result  as  a  new  power  acting  upon  the  second  ma- 
chine, and  so  on. 

141.  An  Example. — A  horse  is  harnessed  to  the  end 
of  a  capstan  bar  (Fig.  29)  at  a  distance  of  5  feet  from 
I,bo  centre  of  the  capstan  barrel  which  is  15  inches  in 
diameter.      The  rope  wound  upon  the   capstan   barrel 
belongs  to  a  system  of  tv.'o  fixed  and  three  movable 
pulleys.     If  the  horse  exerts  a  force  of  500  pounds  and 
we  allow  25  per  cent,  for  loss  by  friction,  what  force 
will  be  exerted  upon  the  building  which  is  being  thus 
qaoved? 


90  NATURAL  PHILOSOPHY.  §  141 

Solution. — The  horse  travels  over  the  circumference  of 
a  circle  10  feet  in  diameter.  In  the  mean  time,  the 
capstan  will  wind  up  a  length  of  rope  equal  to  the  cir- 
cumference of  a  circle  15  inches  (1£  foot)  in  diameter. 
As  the  diameter  or  circumference  of  this  larger  circle  is 
eight  times  as  great  as  the  diameter  or  circumference  of 
the  capstan  barrel,  the  power  moves  eight  times  as  far 
and  eight  times  as  fast  as  the  weight  does.  Therefore 
the  force  exerted  by  the  horse  will  be  increased  eightfold 
and  the  end  of  the  rope  will  be  pulled  with  a  force  eight 
times  500  pounds,  or  4,000  pounds.  This  increased  in- 
tensity of  effort  is  the  effect  of  the  capstan  alone. 

By  drawing  a  sketch  or  diagram  of  the  pulleys,  it  will 
be  seen  that  the  fixed  end  of  the  rope  must  be  attached 
to  the  fixed  block  and  that  there  will  be  six  parts  of  the 
rope  acting  upon  the  movable  block  and,  thus,  upon  the 
weight.  Then  the  pulley  will  increase  the  effect  of  the 
capstan  sixfold. 

4,000  Ib.  x  6  =  24,000  Ib. 
Deduct  i  for  loss  by  friction,     6,000  Ib. 


And  we  have  left    18,000  Ib. 
«8  the  force  exerted  by  the  compound  machine. 

(a.)  The  solution  may  be  simplified  as  follows  •. 
100  3 

JL  *-X-l*-*-6  x     =  18,000  Ib. 


§  144  FRICTION.  91 

142.  What   is  Friction  ?  —  Friction   is   the   re- 
sistance  which  a  moving  body   meets  from   the 
surface  on  which  it  moves. 

143.  The  Cause  of  Friction. — It  is  impossible,  by 
any  known  means,  to  produce  a  perfectly  smooth  sur- 
face.    Even  a  polished  surface  contains  minute  projec- 
tions which  fit  into  corresponding  depressions  on  the 
opposing  surface.      To  produce  motion  of  one  surface 
on   the  other,  these    projections  must    be  lifted   out, 
bent  down  or  broken  off. 

Friction  is  generally  lessened  by  polishing  and  lubri- 
cating the  surfaces  that  move  upon  each  other  and  often 
by  making  the  two  bodies  of  different  material.  The 
axles  of  railway  curs  are  made  of  steel,  the  boxes  in  which 
they  turn  are  made  of  brass,  the  surfaces  are  made  smooth 
and  kept  oiled.  In  spite  of  all  of  these  precautions,  the 
axle  often  becomes  heated  by  friction  to  such  an  extent 
as  to  render  it  necessary  to  stop  the  train. 

144.  Friction  is  a  Transformer  of  Energy.— 
Friction  always  develops  heat :  in  other  words,  it  con- 
verts mechanical  energy  into  a  familiar  form  of  mo- 
lecular  energy.      Whenever  we   find  a  loss  of  power 
through  friction,  we  should  bear  in  mind  that  the  miss- 
ing energy  has  not  been  destroyed.     It  has  simply  been 
transformed  and  still  exists  somewhere  in  the  form  of 
heat. 

Energy,  as  well  as  matter,  is  continually 
changing  its  form  but  it  can  not  be  destroyed 


92 


NATURAL   PHILOSOPHY. 


§145 


145.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


INCLINED    PLANE. 


("  DEFINITION. 


[  LAW. 


WEDGE. 


DEFINITION. 
TWO   USES. 


SCREW. 


DEFINITION. 
LAW. 


COMPOUND    MACHINES ;   RELATION  OF  p  TO 


FRICTION. 


DEFINITION. 
CAUSE. 
REMEDY. 
[BFFBCT 


§  145  EXERCISES.  93 


EXERCISES. 

1.  A  boy  who  can  lift  only  100  pounds  wishes  to  put  a  barrel  ol 
flour  (196  pounds)  into  a  wagon  box  5  feet  above  the  ground.     He 
backs  the  wagon  to  one  end  of  a  plank  20  feet  long  and  weighing 
125  pounds.     Show  that  he  can,  without  help,  use  the  plank  as  an 
inclined  plane  for  his  purpose  and  state  how  much  force  he  exerts 
(a.)  in  getting  the  plank  into  position  and  (6.)  how  much  in  lifting 
the  flour?  Ans.  (a.)  62£  lb.;   (6.)  49  Ib. 

2.  How  long  must  an  inclined  plane  be  that  a  force  of  20  pounds 
may  support  a  weight  of  60  pounds,  one  end  of  the  plane  being 
10  feet  higher  than  the  other  end  ? 

3.  With  apparatus  arranged  as  shown  in  Fig.  33,  the  spring 
balance  reads  15  pounds.     What  is  the  value  of  Wt 

4.  A  given  screw  has  4  threads  to  the  inch.     It  is  worked  by  a 
power  that  moves  around  a  circle  of  40  inches  circumference.     I 
wish  to  exert  a  pressure  of  1,600  kilograms.     How  much  power 
must  1  use  ?  Ans.  10  Kg. 

5.  With  the  screw  described  in  the  last  exercise,  what  pressure 
can  be  exerted  by  a  power  of  30  pounds,  allowing  -fa  of  the  same 
for  friction  1  Ant.  4,320  lb. 


94  NATURAL  PHILOSOPHY.  §  145 


REVIEW     QUESTIONS. 

1.  How  does  the  rising  of  a  man  in  a  small  boat  affect  the  st» 
bility  of  the  boat  ?     Why  ? 

2.  Why  does  a  hand  saw  become  warm  when  it  is  used? 

3.  On  what  two  things  does  momentum  depend ? 

4.  Define  the  term  machine. 

5.  What  advantage  is  gained  by  the  use  of  a  fixed  pulley  ? 

6.  What  is  the  difference  between  adhesion  and  cohesion. 

7.  Tell  what  property  of  matter  is  affirmed  in  the  declaration 
of  "  The  Cloud  " : 

'•  I  pass  through  the  pores  of  the  ocean  and  shores, 
I  change  but  I  can  not  die." 

8.  (a.)  What  two  studies  constitute  physical  science?    (6.)  Of 
what  two  subjects  do  they  treat  ? 

9.  State  the  relative  positions  of  the  point  of  support  and  the 
centre  of  gravity  in  each  of  the  three  kinds  r>r  equilibrium. 

10.  Explain  why  a  "running  jump"  if    onger  than  a  "stand- 
ing" one. 

11.  (a.)  Which  is  the  more  valuable,  a  lump  of  gold  weighing, 
on  a  spring  balance,  one  pound  at  the  surface  of  the  earth  or  a 
lump  that  would  similarly  weigh  as  much  1,000  miles  above  the 
surface  ?    (6.)  Why  would  not  a  lever  balance  (Fig.  23)  answer  for 
the  comparison  as  well  as  a  spring  balance  ? 

12.  If  the  molecules  of  your  body  do  not  touch  one  another, 
why  is  it  that  the  wind  does  aot  blow  you  away  in  the  form  of 
fine  dust  ( 

13  What  force  must  be  overcome  in  order  to  scratch  a  sub- 
stance? 

14.  Dip  a  glass  rod  into  mercury  and  tell  which  is  the  stronger, 
the  adhesion  of  the  liquid  for  the  rod  or  T,he  cohesion  of  the  liquid 
molecules  ? 


§  145  REVIEW   QUESTIONS.  9$ 

15.  Show  that  in  lifting  at  the  end  of  the  plank  mentioned  in 
Exercise  1,  page  93,  the  plank  represented  a  lever  of  the  second 
class,  and  indicate  the  positions  of  P,  F  and  W. 

16.  (a.)  When  water  is  poured  from  a  jar,  it  often  runs  down 
the  inclined  side  of  the  vessel  instead  of  falling  vertically.     What 
force  draws  the  falling  water  from  a  vertical  course?     (b.)  Can 
you  suggest  a  way  to  prevent  such  a  result  ? 

17.  If  a  stone  weighs  10  pounds  at  the  level  of  the  ocean,  how 
much  will  its  weight  measure,  by  a  spring  balance,  1,000  miles 
nearer  the  centre  of  the  earth  ? 

18.  Why  is  it  easier  to  roll  a  sphere  than  a  cube  ? 

19.  What  is  a   foot-pound  ?      A  kilogram-metei  ?      A   horse- 
power? 

20.  Find  the  kinetic  energy  of  a  100-pound  bal)  owiutf  with 
a  velocity  of  2,000  feet  a  second. 


CHAPTER 

LIQUIDS. 


SECTION      I. 
LIQUID      PRESSURE. 

experiment  44.  —  Fill  a  small  bottle  with  water,  hold  a 
Prince  Rupert  drop  in  its  mouth  and 
break  off  the  tapering  end  of  the 
"  drop."  The  whole  "  drop  "  will  be 
instantly  shattered  and  the  force  oi 
the  concussion  transmitted  in  every 
direction  to  the  bottle  which  will  be 
thus  broken. 

These  "drops"  are  not  expensive 
and  may  be  obtained  from  JAMES  W. 
QUEEN  &  Co.,  Philadelphia. 

146.  Transmission  of 
Pressure. — Fluids  transmit 
pressure  in  every  direction, 
upward,  downward,  and 
sidewise  at  the  same  time. 

(a.)  This  property  of  liquids  may 

„        >o  be  illustrated  by  the  apparatus  repre- 

sented in  Fig.  42.      The   globe  and 

cylinder  being  filled  with  water  and  the  several  openings  in  the 
globe  closed  by  corks,  a  piston  is  pushed  down  the  cylinder.  The 
pressure  thus  receded  and  transmitted  by  the  confined  watei 


LIQUID  PRESSURE. 


expels  the  cork  and  throws  a  jet  of  water  from  each  aperture  and 
not  merely  from  the  one  opposite  the  piston. 


(6.)  Figure  43  represents  a  corked  bottle  of 
water.  When  the  cork  is  forced  downward,  it 
exerts  pressure  upon  all  the  water  molecules  in 
contact  with  it.  These  transmit  the  pressure  to 
every  part  of  the  inner  surface  of  the  glass.  Every 
part  of  this  surface  equal  in  area  to  that  of  the 
«nd  of  the  cork  will  be  subjected  to  a  pressure 
like  that  exerted  by  the  cork.  The  riressure  acts 
in  a  direction  perpendicular  to  the  surface  of  the 
gla^s,  as  shown  by  the  arrows  in  the  figure. 

(c.)  It  must  be  remembered  that  fluids  include 
both  aeriform  and  liquid  bodies.  Aeriform  bodies 
are  largely  compressible;  liquids  are  nearly  in- 
compressible. 


147.  Pascal's  Principle. —  Wlien  fluids  are  sub- 
jected to  pressure,  the 
pressure  sustained  by  any 
part  of  the  restraining 
surface  is  perpendicular 
to  it  and  proportional  to 
its  area. 

This  may  be  shown  by  ex- 
periment as  follows : 

Provide  two  communicat- 
ing tubes  of  unequal  sectional 
area.     When  water  is  pourecl 
into  these,  it  will  stand  at 
the  same  height  in  both  tubes.      If  the  water  in  the 
smaller  tube  be  subjected  to  pressure  by  means  of  a 
5 


FIG.  44 


98  NATURAL  PHILOSOPHY.  §  147 

piston,  the  water  will  be  forced  back  into  the  larger  tube. 
To  prevent  this  result,  a  piston  must  be  fitted  to  the 
larger  tube  and  held  there  with  a  force  as  many  times 
as  great  as  the  force  acting  upon  the  other  piston  as  the 
area  of  the  larger  piston  is  times  as  great  as  the  area  of 
the  smaller  one. 

If,  for  example,  the  smaller  piston  have  an  area  of  1 
square  inch  and  the  larger  piston  an  area  of  16  square 
inches,  a  weight  of  1  kilogram  or  ounce  may  be  made  to 
support  a  weight  of  16  kilograms  or  ounces.  Of  course, 
the  weight  here  referred  to  includes  the  weight  of  the 
piston  itself  in  each  case. 

(a.)  It  is  evident  that  in  this  experi- 
ment it  will  be  difficult  to  get  the  pistons 
to  work  without  considerable  friction. 
For  this  reason,  the  experiment  is  some- 
times modified  and  simplified  by  filling 
the  lower  part  of  the  tubes  with  mer- 
cury, which  will  stand  at  the  same  level 
in  both  arms.  If  water  be  poured  into 
the  smaller  tube  it  will  depress  the 
mercury  surface  in  that  tube  ;  16  times 
the  weight  of  water  must  be  poured  into 
the  larger  tube  to  restore  the  two  mer- 
cury surfaces  to  the  same  level. 

148.   Pascal's  Experiment. 

— Pascal  firmly  fixed  a  very  nar- 
row tube  about  30  feet  high  into 
the  head  of  a  stout  cask.  He 
then  filled  the  cask  and  tube  with 
FIG.  45.  water.  The  weight  of  tb;  small 

amount  of  water  in  the  tube  actually  burst  the  cask. 


§  ISO  LIQUID  P". 

149.  The    Hydrostatic    Bellows.  —  The   hydro- 
static bellows  consists  of  two  boards  fastened  to- 
gether by  a  broad  band  of  stout  leather 

and  a  small  vertical  tube  communicat- 
ing with  the  interior. 

In  the  figure,  the  tube  that  bears  the  funnel 
is  to  be  joined  to  the  tube  passing  upward 
from  b.  This  vertical  tube  may  be  filled  with 
water  and  the  pressure  thus  exerted  will  lift 
a  heavy  weight  (e.  g.,  several  boys)  placed 
upon  B. 

If  the 'board,  B,  has  a 
surface  of  66  square  inches 
exposed  to  the  upward 
pressure  of  the  water  in  the 
bellows  and  the  tube  have  a 
sectional  area  of  \  square  FIG.  46. 

inch,  every  pound  of  water  in  the  tube  will  support  264 
pounds  at  B. 

150.  The  Hydrostatic   Press.  —  The  hydrostatic 
press  acts  upon  the  same  principle.     It  is  represented  in 
perspective  by  Fig.  47  and  in  section  by  Fig.  48.     Pres- 
sure is  produced  by  the"  force- pump  A.     The  substance 
to  be  pressed  is  placed  between  K,  the  head  of   the 
piston,  and  an  immovable  plate  M  N.    The  reservoir,  B, 
uiid.the  cylinder  of  the  pump,  are  connected  by  the  tube 
d.     By  the  action  of  the  pump,  the  water  in  the  cylinder 
A  is  subjected  to  pressure  and  this  pressure  is  trans- 
mitted undiminished  to  the  water  in  B.     The  piston,  ' 


100 


XATVRAL  PHILOSOPHY. 


§150 


is  generally  worked  by  a  lever  of  the  second  class,  result- 
ing in  a  still  further  gain  of  intensity  of  power. 

If  the  power-arm  of  the  lever  be  ten  times  as  long 
as  the  weight-arm,  a  power  of  40  pounds  at  the  end  of 


FIG.  47. 

the  lever  will  exert  a  pressure  of  400  pounds  upon  the 
water  in  A. 

If  the  piston  in  A  have  a  sectional  area  of  1  square 
inch  and  the  piston  in  B  have  an  area  of  500  square 


LIQUID   PRESSURE. 


101 


inches,  then  the  pressure  of  400  pounds  exerted  by  the 
small  piston  will  produce  a  pressure  of  400  pounds  x 
500  or  200,000  pounds  upon  the  lower  surface  of  the 
large  piston.  Hence  the  following  rule  : 

Multiply  the  pressure  exerted  by  the  piston  of 
the  pump  by  the  ratio  between  the  sectional 
areas  of  the  two  pistons. 


FIG.  48. 

Experiment  45. — Over  the  opening  of  a  wide  mouthed  bottle 
or  fruit  jar,  tie  a  piece  of  thin  sheet  rubber.  Hold  the  bottle  in  a 
tub  of  water  with  the  mouth  downward,  sidewise  and  upward. 
In  each  case,  the  pressure  of  the  water  will  force  the  rubber  inward. 
Try  the  experiment  at  different  depths  of  water  and  notice  that 
the  pressure  will  vary  with,  the  depth. 

151.  Liquid  Pressure  Due  to  Gravity. —  The 
pressure  exerted  by  liquids,  on  account  of  their  weight, 
may  be  downward,  upward,  or  lateral.  We  shall  now 
briefly  consider  these  three  kinds  of  liquid  pressure, 


102 


NATURAL   PHILOSOPHY. 


152 


Experiment  46.— Into  a  bent  glass  tube,  ABC,  pour  mercury 
(quicksilver)  until  it  covers  the  bend  and  rises  two 
or  three  inches  above  it.  The  mercury  will  stand 
at  the  same  level,  a  c,  in  the  two  branches  of  the 
B  tube.  If  pressure  of  any  kind  be  exerted  upon  the 
surface  of  the  mercury  at  a,  it  will  be  promptly 
shown  by  the  movement  of  the  mercury  and  a 
consequent  difference  in  the  heights  of  the  two 
mercury  columns.  The  greater  the  pressure,  the 
greater  will  be  the  elevation  of  c  above  the  l«vel 
of  a. 

A  common  glass  funnel  or  a  piece  of  glass 
tube  may  be  joined  to  A  by  a  short  piece  of 
snugly  fitting  rubber  tubing.  Suppose  the  funnel 
to  be  thus  connected  and  the  apparatus  supported 
by  any  convenient  means  in  an  upright  position. 
Pour  water  into  the  funnel  and  mark  the  level  of 
the  water  by  a  suspended  weight  or  other  means  ; 
mark  the  level  of  the  mercury  in  the  long  arm  of 
i  11  the  tube  by  a  string  or  small  elastic  band. 

Remove   the   funnel   and  connect  the  straight 

FIG   49  glass   tube   above  A.      Fill    this   with   water  to 

the  same  height   as  before,  as  indicated  by  the 

suspended  weight.     Although  the  quantity  of  water  used  is  much 

less,  the  depression  of  the  mercury  at  a  and  its  elevation  at  c 

will  be  the  same  as  before,  showing  the  downward  pressure  at  a 

to  be  the  same  in  both  cases      Directions  for  bending  glass  will  be 

found  in  Chemistry,  Appendix  4. 

152.  Downward  Pressure. — The  pressure  on  the 
bottom  of  a  vessel  containing  a  liquid  depends 
upon  the  depth  and  density  of  the  liquid  and 
the  area  of  the  bottom. 

The  quantity  of  the  liquid  and  the  shape  of  the  vessel 
niake  no  difference, 


§  154  LIQUID  PRESSURE.  103 

153.  Rule  for  Downward   Pressure.  —  To  find 
the  downward  pressure  on   a  horizontal  surface, 
find  the  weight  of  an  imaginary  column  of  the 
given  liquid,  with  a  base  the  same  as  the  given 
surface   and   an  altitude  the  same  as  the  depth 
of  the  given   surface    below    the   surface   of  the 
liquid. 

NOTE. — A  cubic  foot  of  water  weighs  about  1,000  ounces,  62£ 
pounds  (more  exactly  62.42  pounds). 

154.  Example. — A  cask  has  a  base  of  3  square  feet 
and  a  height  of  2  feet.     A  tube  is  fitted  into  the  upper 
head.    The  cask  is  filled  with  water ;  more  water  is  then 
added  until  the  tube  is  filled  to  a  height  of  3  feet  above 
the  upper  head.     What  is  the  pressure  on  the  lower  head 
of  the  cask  ? 

Solution. — Our  "imaginary  column"  of  water  has  a 
base  of  3  square  feet  and  a  height  of  5  feet.  It  therefore 
has  a  volume  of  15  cubic  feet.  The  weight  of  the 
"imaginary  column"  of  water  would  be  15  times  ^2.42 
pounds  or  936.3  pounds.  This  is  the  pressure  exerted 
upon  the  lower  end  of  the  cask. 

Although  the  quantity  of  water  actually  used  is  less 
than  half  that  of  our  "imaginary  column,"  the  pres- 
sure exerted  by  it  is  the  same  as  explained  in  §  152. 
If  a  liquid  like  alcohol  had  been  used,  the  pressure  would 
have  been  only  about  -fa  of  936.3  pounds,  for  alcohol  is 
only  about  -^  as  heavy  as  water.  If  the  liquid  used  had 
been  mercury,  which  is  13.6  times  as  heavy  as  water,  the 
pressure  would  have  been  13.6  times  936.3  pounds  or 
12,733.68  pounds—  more  than  six  tons. 


104  NATURAL   PHILOSOPHY.  §  *55 

NOTE. — In  all  such  cases  the  pupil  may  be  allowed  to  use  62| 
pounds  as  the  weight  of  a  cubic  foot  of  water  if  he  prefers  to  do 
BO.  That  value  (1,000  ounces)  is  more  easily  remembered. 

Experiment  47.— Make  a  small  hole  in  the  bottom  of  a  tin  fruit 
ean  or  similar  vessel.  Push  the  can  downward  into  water  until 
the  open  mouth  of  the  can  is  "  near  the  water's  edge."  The  liquid 
Will  spurt  upward  through  the  hole  in  a  little  jet. 

155.  Rule  for  Upward  Pressure.— To  find  the 
upward  pressure  on  any  horizontal  surface,  find 
the  weight  of  an  imaginary  column  of  the  given 
liquid  with  a  base   the  same  as  the  given  sur- 
face  and   an   altitude  the  same  as  the  depth  of 
the    given    surface '  below    the    surface    of    the 
liquid. 

156.  Example. — What  will  be  the  upward  pressure 
on  a  horizontal  plate  a  foot  square  when  placed  10  feet 
beneath  the  surface  of  water  ? 

Solution. — The  volume  of  our  "imaginary  column" 
of  water  is  10  cubic  feet.  It  would  weigh  10  times 
62.42  pounds  or  624.2  pounds.  Consequently,  the  up- 
ward pressure  would  be  624.2  pounds. 

157.  Rule  for  Lateral  Pressure. —  To  find  the 
pressure    upon    any    vertical    surface,    find    the 
weight   of  an   imaginary  column   of  the   liquid 
with  a  base  equal  to  the  given  surface   and   an 
altitude  the  same   as  the  depth  of  the  centre  of 
the    given    surface    below    the    surface    of    the 
liquid. 


§  159  LIQUID  PRESSURE.  105 

158.  Example. — A  dam  is  25  feet  high  and  30  feet 
long.     Water  stands  at  a  height  of  20  feet  on  one  side 
of  the  dam.     What  is  the  liquid  pressure  on  the  dam  ? 

Solution. — The  dam  may  be  vertical  or  sloping,  but  in 
either  case,  the  surface  that  we  have  to  consider  is  20  feet 
high  and  30  feet  long.  The  depth  of  its  centre  below 
the  surface  of  the  water  is  10  feet.  Our  "imaginary 
column  "  will,  therefore,  have  a  volume  of  6,000  cubic 
feet  and  a  weight  of  374,520  pounds.  The  pressure  will, 
therefore,  be  374,520  pounds. 

159.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 

TRANSMITTED  EQUALLY  IN  ALL  DIRECTIONS. 

PASCAL'S  EXPERIMENT. 

EFFECT  ON  RESTRAINING  SURFACE.  \  HYDROSTATIC  BELLOWS 

HYDROSTATIC  PRESS. 


f  DOWNWARD. 

PRODUCED  By  GRAVITY J  UPWARD. 

I  LATERAL. 


106  NATURAL  PHILOSOPHY.  §  159 


EXERCISES. 

1.  Referring  to  Fig.  48,  suppose  the  area  of  the  end  of  piston,  a, 
to  be  1  square  inch  and  that  of  piston,  C,  to  be  10,000  square 
jaches  and  the  two  arms  of  the  lever,  0,  to  be  1  foot  and  10  feet 
respectively,  what  pressure  at  K  will  be  produced  by  a  pressure 
of  100  pounds  by  the  operator?  Am.  10,000,000  Ib. 

2.  Where  will  water  issue  from  a  water  pipe  with  the  greater 
force,  in  the  basement  or  near  the  top  of  a  house  ?     Why  ? 

3.  Find  the  pressure  on  a  dam  25  feet  long,  the  water  being  10 
feet  deep  ?  Am.  78025  Ib. 

4.  A  tank  has  a  base  6  feet  by  8  feet.     What  is  the  pressure  on 
that  base  when  the  water  in  the  tank  is  4  feet  deep  ? 

Ans.  11984.64  Ib. 

5.  A  tank  has  a  base  2  meters  by  3  meters.    What  is  the  pres 
sure  on  that  base  when  the  water  in  the  tank  is  1.5  meters  deep  ? 

Ant.  9000  Kg. 


§  l6l  EQUILIBRIUM.  107 

SECTION     II. 

EQUILIBRIUM.— BUOYANCY. 

160.  Equilibrium  of  Liquids.  —  A  liquid  of  small 
surface  is  said  to  be  level  when  all  the  points  of  its  sur- 
face are  in  the  same  horizontal  plane.     The  central  idea 
is  expressed  in  the  familiar  saying,   water  seeks  Us 
level.     This  is  true  whether  the  liquid  be  placed  in  a 
single  vessel  or  in  several  vessels  that  communicate  with 
each  other. 

Experiment  48. — Incline  a  tea-pot  that  is  nearly  full  of  water 
until  the  liquid  begins  to  run  out  at  the  spout.  Notice  that  the 
end  of  the  spout  and  the  water  surface  in  the  vessel  are  at 
the  same  level. 

161.  Communicating  Vessels. — When  a  liquid  is 
placed  in  one  or  more  of  several  vessels  communicating 
with  each  other,  it  will  not  come  to  rest  until  it 
stands  at  the  same  height  in  all  of  the  vessels. 

Experiment  49. — From  one  end  of  a  scale-beam,  suspend  a 
cylindrical  bucket  of  metal,  b,  and  below  that  a  solid  cylinder,  a, 
winch  accurately  fits  into  the  bucket.  Counteqwise  with  weights 
in  the  opposite  scale-pan.  Then  place  a  vessel  of  water,  as  shown 
in  Fig.  50,  so  as  to  immerse  «  and  the  counterpoise  will  descend, 
showing  that  a  has  lost  some  of  its  weight.  Carefully  fill  6  with 
water.  It  will  hold  exactly  the  quantity  displaced  by  a.  Equili- 
brium will  be  restored.  The  bucket  and  cylinder  may  be  had 
of  dealers  in  philosophical  apparatus. 


108 


NATURAL  PHILOSOPHY. 


§i6i 


Experiment  50. — Insert  a  short  spout  in  the  side  of  a  vessel  (as 
«  tin  fruit-can)  about  an  inch  below  the  top.  Fill  the  vessel  with 
water  and  let  all  above  the  level  of  the  spout  escape.  This  is  to 
replace  the  vesrel  of  water  in  which  a  (Fig.  50)  is  immersed. 
Instead  of  the  bucket,  ft,  use  a  cup  placed  on  the  scale-pan.  Instead 
i>f  a,  use  any  convenient  solid  heavier  than  water,  as  the  fragment 
of  a  stone,  which  may  be  suspended  by  a  horse-hair  or  a  fine 
thread.  Counterpoise  the  cup  and  stone  in  the  air.  Immerse  the 


FIG.  50. 

stone  in  the  water  and  catch,  in  any  convenient  vessel,  every  drop 
of  water  that  overflows.  This  will  be  the  fluid  that  the  solid 
displaces.  The  equilibrium  is  destroyed,  but  may  be  restored  by 
pouring  the  water  just  caught  into  the  cup  on  the  scale  pan.  It 
may  sometimes  be  necessary  to  make  allowance  for  the  small 
quantity  of  water  that  adheres  to  the  cup  in  which  the  overflow 
was  caught,  but  with  a  good  balance  and  good  work  the  result  will 
clearly  show  the  truth  of  Ardiiinetfw?  Principle. 


§  163  BUOYANCY.  109 

162.  Archimedes'  Principle. — It  is  a  familiar  fact 
«;hat  a  person  may  easily  raise  to  the  surface  of  the  water 
a  stone  which  he  can  not  lift  any  further.  When  an  arm 
or  leg  is  lifted  out  of  the  water  of  a  bath-tub,  it  suddenly 
feels  heavier.  Many  such  facts  are  explained  by  the 
important  principle  here  given  : 

The  loss  of  weight  of  a  body  immersed  in  a 
fluid  equals  the  weight  of  the  fluid  which  it 
displaces. 

Experiment  51. — Place  the  tin  vessel  with  a  spout,  mentioned 
in  Experiment  50,  upon  one  scale-pan,  and  fill  it  with  water,  some 
of  which  will  overflow  through  the  spout.  When  the  spout  has 
ceased  dripping,  counterpoise  the  vessel  of  water  with  weights  in 
the  other  scale-pan.  Place  a  floating  body  on  the  water.  This 
will  destroy  the  equilibrium  but  water  will  overflow  through  the 
epout  until  the  equilibrium  is  restored.  This  shows  that  the 
floating  body  has  displaced  its  own  weight  of  water. 

Experiment  52. — Place  a  fresh  egg  in  a  vessel  of  fresh  water; 
it  is  a  little  heavier  than  the  water  and  will  sink.  Place  it  in  salt 
water ;  it  is  a  little  lighter  than  the  brine  and  will  float.  Carefully 
jiour  the  fresh  water  on  the  salt  water  in  a  tall,  narrow  vessel  like 
that  shown  in  Fig.  162.  Place  the  egg  in 
the  water;  it  will  descend  until  it  reaches 
a  layer  of  the  liquid  with  a  density  like  its 
own  and  there  it  will  float. 

163.  Floating  Bodies.  —  A 
floating  body  displaces  its  own 
weight  of  the  fluid  on  which  U 
floats. 

FIG.  61.  Th-g  may  be  g]lown  experimentally 

by  filling  a  vase  with  water.     When  a  floating  body  is 


110  NATURAL   PHILOSOPHY.  §  1 63 

placed  on  the  surface,  the  water  displaced  will  overflow 
and  may  be  caught.  The  water  thus  caught  will  weigh 
the  same  as  the  floating  body. 

(a.)  If  a  body  placed  on  the  surface  of  a  fluid  can  not  displace 
its  own  weight  of  the  fluid,  it  will  sink  and  not  float.  The  buoy- 
ant effect  of  the  fluid  is  then  less  than  the  weight  of  the  body. 
Sometimes  a  heavy  substance  has  such  a  shape  that  it  will  dis- 
place enough  of  a  lighter  fluid  to  float  thereon.  For  this  reason, 
an  iron  kettle  or  an  iron  ship  will  float  on  water,  although  iron  is 
several  times  heavier  than  water.  In  all  such  cases,  the  buoy- 
ant effect  of  the  fluid  equals  the  weight  of  the  floating  body, 

(6.)  Just  as  the  gravity  of  a  body  may  be  considered  as  acting 
upon  a  single  point  called  the  centre  of  gravity,  so  the  buoyant 
effort  of  a  fluid  may  be  considered  as  act 
ing  upon  a  single  point  called  the  centre 
of  buoyancy.    The  centre  of  buoyancy 
is  situated  at  the  centre  of  gravity  of 
the  displaced  fluid. 

(c.)  The  centre  of  buoyancy  may  be 
considered  the  point  of  support  of  the 
floating  body  and  the  principles  of  §§  65-  FlG-  53- 

C8  applied  anew.      For  example,  if  the 

boat's  centre  of  buoyancy  be  at  C  and  its  centre  of  gravity  b« 
at  b  (as  it  may  be  if  its  occupant  is  sitting  down),  the  boat 
will  be  in  stable  equilibrium,  while  if  its  centre  of  gravity  be  at 
B  (as  it  may  be  if  its  occupant  is  standing  up),  the  boat  will  b« 
in  dangerously  unstable  equilibrium. 


RECAPTTULA  r/O.V. 


Ill 


164.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


(XI 

g al 


I  I 
11 


H 


11.3  NATURAL  PHILOSOPHY.  <j  164 


EXERCISES. 

1.  Where  are  water  pipes  of  uniform  strength  connected  with 
the  same  reservoir  more  likely  to  burst,  on  a  hill  or  in  a  valley  1 
Why? 

2.  What  weight  of  water  will  a  50-pound  canoe  displace? 
8.  What  weight  of  water  will  a  cubic  foot  of  iron  displace  ? 

4.  How  much  weight  will  a  cubic  foot  of  lead  lose  when  it  is 
placed  in  water  ? 

5.  Do  you  know  anything  about  the  personal  history  of  Arch\- 
medes? 

6.  How  much  water  will  a  board  displace,  (a.)  when  it  floats  on 
the  surface ?    (6.)  When  it  is  held  under  the  surface? 

7.  When  is  a  boat  in  stable  equilibrium  ? 

8.  In  lifting  a  pail  of  water  from  a  cistern,  it  does  not  seem 
ieavy  until  it  is  raised  out  of  the  water.     Why  is  this  ? 

9.  Why  is  it  difficult  to  stand  in  water  reaching  to  your  chin  ? 


§  107  SPECIftC  GRAVITY.  113 

SECTION     III, 

SPECIFIC    GRAVITY. 

165.  What  is  Specific  Gravity  ?  —  The  specific 
gravity  of  a  body  is  tlie  ratio  between  its  weight 
and  the  weight  of  a  like  volume   of  some   other 
substance  taken  as  a  standard. 

It  is  an  abstract  number  and  shows  how  many  timei 
the  weight  of  a  body  will  contain  the  weight  of  the  same 
volume  of  some  other  substance  that  is  taken  as  a 
standard. 

166.  Standards  of  Specific  Gravity.— For  solids 
und   liquids,   the   standard   adopted   is   distilled 
water  at  a  temperature  of  Jf  @->  or  39.2°  F. 

For  aeriform  bodies,  the  standard  is  air  or 
hydrogen. 

167.  Elements  of  the  Problem.  —  For  solids  or 
liquids,  the   dividend  is  the  weight  of  the  given 
body ;  the  divisor  is  the  weight  of  the  same  bulk 
of  water;    the    quotient,    which    is   an    abstract 
nuniber,  is  the  specific  gravity. 

The  weight  of  the  same  bulk  of  water  is  found  some- 
times in  one  way  and  sometimes  in  another  but,  in  every 
case,  it  is  tlie  divisor.  By  grasping  and  keeping  thi? 
idea,  you  will  avoid  much  possible  confusion.  Of  course, 
when  any  two  of  these  three  are  given,  the  third  can  be 
found. 


114 


NATURAL  PHILOSOPHY. 


§168 


168.  To  Find  the  Specific  Gravity  of  Solids.— 
The  most  common  method  of  finding  the  specific  gravity 
of  a  solid  heavier  than  water,  is  to  find  the  weight  of 
the  body  in  the  air  (=  W),  then  suspend  the  body  by  a 
light  thread  and  find  its  weight  in  water  (=  W),  and 
divide  the  weight  of  the  body  in  air  by  the  weight  of  the 
same  bulk  of  water  (§  162,  Archimedes'  Principle). 

Sp.  Or.^ 


(a.)  The  method  is  illustrated  by  the  following  example  ; 
Weight  of  substance  in  air  =  58£  oz. 

Weight  of  substance  in  water         =  51    oz.  (Fig.  54). 

Weight  of  equal  bulk  of  water 
Specific  gravity  —  58  1  oz.  -*-  7|  oz. 


§170  SPECIFIC   GRAVITY.  115 

169.  To  Find  the  Specific  Gravity  of  Liquids. 
— The  principle  is  unchanged.  A  sirhple  method  is  as 
follows: 

Weigh  a  flask  first  empty;  next,  full  of  water;  then, 
full  of  the  given  liquid.  Subtract  the  weight  of  the 
empty  flask  from  the  other  two  weights ;  the  results 
represent  the  weights  of  equal  volumes  of  the  given 
substance  and  of  the  standard.  Divide  as  before. 

(a.)  A  flask  of  known  weight,  graduated  to  measure  100  or  1,000 
grams  or  grains  of  water  is  called  a  specific  gravity  fla&k.  Its  use 
avoids  the  first  and  second  weighings  above  mentioned  and  simpli- 
fies the  work  of  division. 

(6.)  The  specific  gravity  of  a  liquid  may  he  easily  determined 
as  follows  : 

Find  the  loss  of  weight  of  any  insoluble  solid  in  water  and  in 
the  given  liquid.  Remember  (§  162)  what  these  two  losses  repre. 
sent.  Di vide  as  before.  The  solid  used  is  called  a  specific  gravity 
bulb. 

(c.)  The  determination  of  the  specific  gravity  of  gases  presents 
many  practical  difficulties  which  can  not  be  considered  in  thla 
place. 

170.  Recapitulation. — To  be  amplified  by  the  pupiJ 
for  review. 

DEFINITION. 

_  STANDARDS. 

SPECIFIC 

DIVIDEND    AND   DIVISOR. 
GRAVITY. 

OF   SOLIDS. 


(  SPECIFIC  GRAV 

OF    LIQUIDS \ 

(  SPECIFIC  GRAV 


vrrv  FLASK 
ITY  BULB. 


116  NATURAL  PHILOSOPHY.  §  170 


EXERCISES. 

1.  A  body  weighs  150  pounds  in  air  and  100  pounds  in  water, 
What  is  its  specific  gravity  ? 

2.  A  body  weighs  75  ounces  in  air  and  60  ounces  in  water. 
What  is  the  weight  of  an  equal  bulk  of  water? 

3.  Lead  has  a  specific  gravity  of  about  11 J.     Mercury  is  a  liquid 
having  a  specific  gravity  of  about  luf .     Will  lead  sink  in  mercury? 

4.  Sulphuric  acid  has  a  specific  gravity  of  1.8.     Will  a  glass 
ball  lose  more  weight  in  the  acid  than  it  will  in  water  ? 

5.  In  which  will  a  body  weigh  more,  in  fresh  water  having  a 
specific  gravity  of  1  or  in  sea  water  having  a  specific  gravity  of 


6.  Is  it,  then,  easier  for  a  person  to  float  in  fresh  water  than  it 
JB  in  sea  water  ? 

7.  A  piece  of  brass  weighing  41.9  ounces  was  placed  in  a  vessel 
full  of  water.    The  overflowing  water  was  caught  and  found  to 
weigh  5  ounces.    What  is  the  specific  gravity  of  brass  ? 

An*.  8.38 

8.  A  liter  bottle  holds  1,000  grams  of  water  or  800  grams  of 
alcohol.    What  is  the  specific  gravity  of  alcohol  ? 

9.  Let  each  pupil  experimentally  determine  the  specific  gravity 
of  some  solid  that  will  sink  in  water. 


§  174  HTDROKINETIC&  117 


SECTION      IV. 

HYDROKINETICS. 

171.  The  Flow  of  Liquids  through  Horizontal 
Pipes. — When  liquids  from  a  reservoir  are  made  to  flow 
through  pipes  of  considerable  length,  the  discharge  is 
far  less  than  that  due  to  the  head.     The  diminution  is 
chiefly  owing  to  friction  against  the  sides  of  the  pipe. 

The  "head"  is  the  vertical  distance  from  the  centre 
of  the  orifice  to  the  surface  of  the  liquid. 

172.  The   Flow  of  Rivers.  —  The  friction  of  a 
stream  against  its  solid  bed  fortunately  retards  the  ve- 
locity of  the  water.     Otherwise  the  velocity  of  the  cur- 
rent at  the  mouth  of  a  river,  whose  head  is  elevated 
1,000  feet  above  its  mouth,  would  be  about  170  miles  per 
hour.     Such  a  current  would  be  disastrous  beyond  de- 
scription.    The  ordinary  river  current  is  from  three  to 
five  miles  per  hour. 

173.  Water-power. — Water  may  be  used  to  turn  a 
wheel  and  thus  move  machinery  by  its  weight,  the  force 
of  the  current  or  both. 

174.  The  Turbine  Wheel.— The  turbine  wheel,  of 
which   there  are  many  varieties,  is  the  most  effective 
water-wheel  known. 


118 


NATURAL   PHILOSOPHY. 


§174 


FIG.  54. 

(a.)  Figure  54  represents  one  form  in  perspective  and  in  hori- 
zontal section  through  the  centre  of  the  wheel  and  case  complete. 
The  wheel,  B,  and  the  enclosing  case,  D,  are  placed  on  the  floor  of 
a  penstock  wholly  submerged  in  water,  under  the  pressure  of  a 
considerable  head.  The  water  enters,  as  shown  by  the  arrows, 
through  openings  in  D,  which  are  BO  constructed  that  it  strikes 
the  buckets  of  B  in  the  direction  of  greatest  efficiency. 

(b.  After  leaving  the  buckets,  the  "  dead  water "  escapes  from 
the  central  part  of  the  wheel,  sometimes  by  a  vertical  draft  tube, 
best  made  of  boiler-iron.  The  weight  of  the  water  in  this  tube 
increases  the  velocity  with  which  the  water  strikes  the  buckets. 

(c.)  A  central  shaft,  A,  is  carried  by  the  wheel  and  communi 
cates  its  motion  to  the  machinery  above.  The  wheel  itself  rests 
upon  a  central  pivot  carried  by  cross-arms  from  the  bottom  of  the 
outer  case.  The  case,  D,  is  covered  with  a  top,  T,  which  protects 
the  wheel  from  the  vertical  pressure  of  the  water.  The  axis  of 
the  wheel  passes  through  the  centre  of  this  cover. 

175.  The  Overshot  Wheel.  —  In  the  overshot 
wheel,  the  water  falls  into  buckets  at  the  top  and,  bv 


HTDK  O  KINETICS. 


119 


its  weight  aided  by  the  force  of  the  current,  turns  the 
wheel.  As  the  buckets  are  gradually  inverted,  the  water 
is  emptied  aud  the  load  thus  removed  from  the  other 
side  of  the  wheel. 


PIG.  55. 

Such  wheels  require  not  much  water  but  a  consider- 
able fall.  It  is  said  that  they  have  been  made  nearly 
100  feet  in  diameter.  The  water  is  led  to  the  top  of 
the  wheel  by  a  sluice,  8  V.  The  power  may  be  com- 
municated to  the  machinery  by  the  shaft,  A,  or  by  the 
rim  of  the  wheel,  as  at  c,  according  to  the  desire  for 
intensity  of  power  or  for  velocity.  The  water  supply 
may  be  controlled  by  a  gate  at  F. 

176.  The  Breast  Wheel.  — In  the  breast  wheel, 
the  water  acts  upon  float  boards  fixed  perpendicular  to 
the  circumference.  The  stream  being  received  at  or 
near  the  level  of  the  axis,  both  the  weight  of  the  water 
and  the  force  of  the  current  may  be  turned  to  account. 


120 


NATURAL  PHILOSOPHY. 


§177 


FIG.  56. 

177.  The  Undershot  Wheel. — In  the  undershot 
wheel,  the  stream  strikes,  near  the  bottom  of  the  wheel, 
against  a  few  float  boards.  It  is  turned  by  the  force  of 
the  current. 


FIG.  57. 

NOTE.— In  point  of  efficiency,  these  wheels  rank  in  the  ordei 
ibove  given, 


§177 


REVIEW  QUESTIONS. 


121 


REVIEW     QUESTIONS. 

1.  In  what  direction  do  liquids  at  rest  exert  pressure? 

2.  Upon  what  three  things  does  the  pressure  of  a  liquid  on  the 
bottom  of  the  vessel  holding  it  depend  ? 

3.  How  may  a  little  water  be  made  to  exert  a  great  pressure  ? 

4.  Show  two  ways  in  which  a  small  weight  may  be  made  to 
balance  a  heavy  one? 

5.  State  two  points  of  difference  between  solids  and  liquids? 

6.  If  a  pendulum  of  given  length  swings  through  an  arc  five 
inches  long  in  one  second,  how  long  will  it  take  the  same  pendulum 
to  swing  through  an  arc  of  ten  inches  ? 

7.  Show  that  the  appara- 
tus represented  in  Fig.  58 
is  a  lever,  tall  of  what  class 
it  is  and  indicate  the  posi- 
tions for  P,  W  and  F. 

8.  Define  malleability. 

9.  In  what  direction  do  fluids  transmit  pressure  with  the  great- 
est facility? 

10.  State  the  principle  that  underlies  the  action  of  the  hydro- 
static press  ? 

11.  What  was  Pascal's  experiment?    Find  out  who  Pascal  was. 

12.  It  is  stated  that  the  specific  gravity  of  silver 
is  about  ten.     This  means  ten  of  what  f 

13.  What  is  the  velocity  of  an  ordinary  river 
current  ? 

14.  Describe  the  hydrostatic  bellows.    What  is 
meant  by  the  centre  of  buoyancy  ? 

15.  Describe  the  use  of  the  specific  gravity  bulb. 

16.  Experiment    shows    that    it   is    difficult  to 
balance  a  lead  pencil  on  the  finger.      Why   is  it 
e:isier  thus  to  balance  it  with  the  additional  load 
of  two  penknives,  as  shown  in  Fig.  59  ? 


-  58- 


FIG.  59. 


CHAPTER    V. 

PNEUMATICS. 


SECTION      I. 

THE    ATMOSPHERE    AND    ATMOSPHERIC 
PRESSURE. 

178.  What    is    Pneumatics  ?  —  Pneumatics  is 
that  branch   of  Physics   which   treats   of  aeri- 
form bodies,  their   mechanical  properties   and 
the  machines  by  which  they  are  used. 

179.  Tension  of  Gases. — However  small  their 
quantity,  gases  always  fill  the  vessels  in  which 
they  are  held. 

If  a  bladder  or  India  rubber  bag, 
partly  filled  with  air  and  having 
the  opening  well  closed,  be  placed 
under  the  receiver  of  an  air-pump 
(§  187),  the  bladder  or  bag  will  be 
fully  distended,  as  shown  in  the 
figure,  when  the  air  surrounding 
the  bladder  is  pumped  out. 

The  flexible  walls  are  pushed  out 
by  the  air  confined  within.  This 
tendency  is  called  elastic  force  or  tension,. 


FIG.  60. 


§  l8l  ATMOSPHERIC  PRESSURE.  123 

We  may  imagine  the  countless  molecules  of  air  in  the 
bladder  or  bag  as  being  in  constant  motion  and  continu- 
ally striking  against  the  walls  that  confine  them.  These 
molecular  blows  push  the  walls  outward  and  their  total 
effect  constitutes  tension. 

This  conception  is  generally  referred  to  as  the 
Kinetic  Ttieory  of  Gases  (§  43;  a). 

180.  The  Type. — As  water  was,  for  obvious  reasons, 
taken  as  the  type  of  liquids,  so  atmospheric  air  will 
be  taken  as  the  type  of  aeriform  bodies. 

Whatever  mechanical  properties  are  shown  as  be- 
longing to  air  may  be  understood  as  belonging  to  all 
gases. 

181.  Weight  of  Air. — Being  a  form  of  matter,  air 
has  weight.      This  may  be  shown  by  experiment.      A 
hollow  globe  of  glass  or  metal,  having  a  capacity  of  sev- 
eral quarts  or  liters  and  provided  with  a  stop-cock,  is 
carefully  weighed  on  a  delicate  balance.     The  air  is  then 
removed  from  the  globe  by  an  air-pump,  the  stop-cock 
closed  and   the  empty  globe  weighed  carefully.      The 
second  weight  will   be  less  than   the  first,   the  differ- 
ence  between  the  two   being  the  weight  of    the    air 
removed. 

(a.)  Under  ordinary  conditions,  a  cubic  inch  of  air  weighs  about 
0.31  grains.  A  liter  of  air  weighs  about  1.293  grams,  being  thus 
about  T^5  as  heavy  as  water. 

(6.)  Measure  the  length,  breadth  and  height  of  your  school- 
room and  find  the  weight  of  the  air  that  it  contains. 


124  NATURAL  PHILOSOPHY.  §  l8l 

Experiment  53.— Fill  a  tumbler  with  water,  place  a  slip  of 
thick  paper  over  its  mouth  and  hold  it 
there  while  the  tumbler  is  inverted :  the 
water  will  be  supported  when  the  hand 
is  removed  from  the  card,  as  is  shown  in 
Fig.  61. 


Experiment  54.— Plunge  a  small  tube, 
or  a  tube  having  a  small  opening  at  the 
lower  end,  into  water,  cover  the  upper 
end  with  the  finger  and  lift  it  from  its 
bath.  The  water  is  kept  in  the  tube  FIO.  61. 

by  the  upward  pressure  of  the  atmos- 
phere. Remove  the  finger  and  the  downward  pressure  of  the  at- 
mosphere, which  was  previously  cut  off,  will  counterbalance  the 
upward  pressure  and  the  water  will  fall  by  its  own  weight.  Such 
a  tube,  called  a  pipette,  is  much  used  for  transferring  small  quan- 
tities of  liquids  from  one  vessel  to  another. 

Experiment  55. — Make  a  "sucker"  of  a  circular  piece  of  thick 
leather  and  fasten  a  string  to  its  middle.  Soak 
it  thoroughly  in  water  and  firmly  press  it  upon 
a  flat  stone  to  drive  out  all  air  from  between  the 
leather  and  the  stone.  Pull  the  string  gently  so 
that  a  vacuum  may  be  formed,  as  shown  in  Fig. 
62.  If  the  stone  be  not  too  heavy,  it  may  be 
lifted  by  the  string.  We  shall  soon  see  that,  in 
reality,  the  stone  is  pushed  up  by  the  air 
instead  of  being  pulled  up  by  the  string. 

Experiment  56.— Suck  the  air  from  the  hol- 
low stem  of  a  common  key  and  quickly  press  the 
FIG.  62.  OP6"  end  of  tne  stem  against  the  lip  where  it 

will  be  held  by  the  pressure  of  the  air. 

Experiment   57.— For  a  few  cents  you  can  buy  a  four  inch 
test  tube  of  a  dealer  in  chemical   glass   ware  (see   Chemistry, 


AfMOSPBERtC  PRESSURE. 


125 


Appendix  7).  Holding  it  by  the  open  end,  heat  the  tube  quite  hot 
in  the  flame  of  a  lamp  or  candle.  When  the  heat  has  expanded 
the  air  and  driven  part  of  it  out  of  the  tube,  press  the  mouth  of 
the  tube  against  the  fleshy  part  of  the  hand  or  thumb  where  it 
will  be  held  by  atmospheric  pressure. 

Experiment  58.  — Vary  the  last  experiment  by  placing  the 
tube,  mouth  downward,  in  a  saucer  of  water.  Atmospheric 
pressure  will  force  water  upward  into  the  tube. 

Experiment  59.— Secure  an  empty  tin  fruit  can  with  a  hole 
about  two  inches  in  diameter 
in  one  end.  Smoothly  stretch 
a  piece  of  clean  mosquito  net- 
ting over  this  end  of  the  can, 
as  shown  in  Fig.  63.  Fill 
the  can  with  water,  place  a 
piece  of  writing  paper  over 
the  mosquito  netting  and 
hold  it  there  while  you  invert 
the  can.  Draw  the  pa  per  hor- 


Fio.  63. 
meshes  of  the  mosquito  netting. 


izontally  from  the  end  of  the 
can.  Atmospheric  pressure 
will  prevent  the  water  from 
running  out  through  the 


Experiment  60.— With  a  small  nail,  punch  a  hole  in  the 
middle  of  the  closed  end  of  the  can  used  in  the  last  experiment. 
Repeat  that  experiment,  keeping  the  nail-hole  covered  by  the 
forefinger,  as  shown  in  Fig.  63.  Remove  the  finger  for  an  instant, 
quickly  covering  the  hole  again.  When  the  atmosphere  has  a 
chance  to  press  downward  through  the  nail-hole  upon  the  water 
in  the  can,  the  water  runs  out  through  the  mosquito  netting. 
When  this  opportunity  is  removed  by  closing  the  nail  hole,  the 
water  is  held  in  the  can  by  the  upward  pressure  of  the  at- 
mosphere. 


126  XATUtlAL  PHILOSOPHY.  §  l82 

182.  Atmospheric    Pressure.  —  The   atmosphere 
exerts  a  great  pressure  upon  the  surface  of  the  earth  and 
all  bodies  found  there.     This  atmospheric  pressure  de- 
creases as  we  ascend  from  the  earth's  surface. 

The  weight  of  a  column  of  air  one  inch  square  and 
extending  from  the  sea-level  to  the  upper  limit  of  the  at- 
mosphere is  about  fifteen  pounds;  a  similar  column,  a 
centimeter  square,  weighs  about  one  kilogram. 

We  express  this  by  saying  that  the  atmospheric 
pressure  at  the  sea-level  is  fifteen  pounds  to  the 
square  inch,  or  one  kilogram  to  the  square  cen- 
timeter. 

(a.)  Several  illustrations  of  atmospheric  pressure  will  be  given 
after  we  have  considered  the  air-pump. 

183.  Torricelli's  Experiment.  —  The  intensity  of 
atmospheric  pressure  may  be  measured  as  follows : 

Take  a  glass  tube  a  yard  long,  about  a  quarter  of  an 
inch  in  internal  diameter.  Close  one  end  and  fill  the 
tube  with  mercury.  Cover  the  other  end  with  the 
thumb  or  finger  and  invert  the  tube,  placing  the  open 
end  in  a  bath  of  mercury.  Upon  removing  the  thumb, 
the  mercury  will  sink  and  come  to  rest  at  a  height  of 
about  30  inches,  or  7GO  millimeters,  above  the  level  of 
the  mercury  in  the  bath. 

The  apparatus  used,  when  properly  graduated,  becomes 
?.  barometer. 

184.  Pascal's  Experiments.— Pascal  repeated  Tor- 
ricelli's experiment  on  the  top  of  a  mountain  and  found 
that  the  mercury  column  was  three  inches  shorter,  show- 


186 


ATMOSPHERIC   PRESSURE. 


127 


ing  that  as  the  weight  of  the  atmospheric  column  dimin 
ishes,  the  supported  column  of  mercury  also  diminishes. 

He  then  took  a  tube  forty  feet  long,  closed  at  one  end. 
Having  filled  it  with  water,  he  inverted  it  over  a  water 
bath.  The  water  in  the  tube  came  to  rest  at  a  height 
of  34  feet.  The  water  column  was  13.G  times  as  high  as 
the  mercury  column,  but  as  the  specific  gravity  of  mer- 
cury is  13.6,  the  weights  of  the  two  columns  were  equaL. 

Experiments  with  still  other  liquids  gave  correspond- 
ing results,  all  of  which  strengthened  the  theory  that 
the  supporting  force  is  due  to  the  weight  of  the  atmos- 
phere and  left  no  doubt  as  to  its  correctness. 

185.  Pressure   Measured  in  Atmospheres. — A 

gas  or  liquid  which  exerts  a  force  of  15  pounds  upon  a 
square  inch  or  one  kilogram  upon  a  square  centimeter  of 
the  restraining  surface  is  said  to  exert  a  pressure  of  one 
atmosphere.  A  pressure  of  60  pounds  to  the  square 
inch,  or  4  kilograms  to  the  square  centimeter  would  bo 
called  a  pressure  of  4  atmospheres. 

186.  Recapitulation.— To  be  amplified  by  the  pupil 
for  review. 


DEFINITION. 


TENSION. 


TYPE  IS  AIR. 


PRESSURE. 


Per  square  inch. 

Per  square  centimeter. 

Measured  in  atntospkeru. 

Torricelli. 

Paseat. 


128  NATURAL  PHILOSOPHY.  §  l86 


EXERCISES. 

1.  The  downward  pressure  of  the  atmosphere  on  the  hottom  of 
an  ordinary  wooden  pail  is  about  a  ton.     Why  is  not  the  bottom 
forced  out  and  how  can  any  person  carry  the  pail  ? 

2.  Suppose  a  bottle  to  be  tightly  corked  at  the  top  of  a  high 
mountain,  carried  to  the  sea-level  and  opened  with  its  mouth  under 
water.     Would  air  bubble  out  or  water  rush  in  ? 

3.  What  is  the  pressure  on  one  side  of  a  window  2  feet  by 
6  feet?  AM.  25920  Ib. 

4.  What  is  the  pressure  on  one  side  of  a  door  1  meter  by  2 
meters  ? 

5.  Why  is  a  barometer  tube  closed  at  the  top  ? 

6.  Why  is  a  barometer  tube  not  closed  at  the  bottom  ? 

7.  When  the  barometer  stands  at  28  inches,  at  what  height 
would  the  water  in  the  tube  of  Pascal's  experiment  come  to  rest  ? 

8.  A  steam-boiler  was  tested  "  at  a  pressure  of  10  atmospheres/ 
What  does  this  mean? 


§187 


AIR-PUMPS. 


SECTION     II. 

AIR-PUMPS.— LIFTING    AND    FORCE-PUMPS.— 
SIPHON. 

187.  The  Air-Pump.  —  The  air-pump  is  an  in- 
strument for  removing  air  from  a  closed  vessel. 

The  essential  parts  are  shown  in  section  by  Fig.  64. 


FIG.  64. 

The  vessel,  R,  is  called  a  receiver.  It  fits  accurately 
upon  a  horizontal  plate,  through  the  centre  of  which  is 
an  opening  communicating  with  a  cylinder,  C,  by  means 
of  a  bent  tube,  t.  An  accurately  fitting  piston  moves  in 
the  cylinder.  At  the  junction  of  the  bent  tube  with  the 
cylinder  and  in  the  piston,  are  two  valves,  v  and  v', 


130 


NATURAL   PHILOSOPHY. 


§187 


opening  from  the  receiver  but  not  toward  it.  When  the 
piston  is  raised,  v'  closes  and  the  atmospheric  pressure 
is  removed  from  v.  The  tension  of  the  air  in  R  opens  v. 
The  air  which  was  in  R  and  t  expands  and  fills  R,  t  and 
C.  When  the  piston  is  pushed  down,  v  closes,  v'  opens 
and  the  air  in  C  escapes  from  the  apparatus. 

NOTE. — A.  person  having  an  air-pump  has  the  means  of  per- 
forming  almost  numl>erless  experiments,  some  amusing  and  all 
instructive.  A  cheap  and  efficient  air-pump  may  be  bought  of 
JAMES  W.  QUEEN  &  Co.,  Philadelphia.  Several  experiments 
with  the  air-pump  are  given  below.  Others  will  be  found  in 
The  Element*  of  Natural  Philosophy. 

Experiment  61.— The  hand-glass  is  a  receiver  open  at  both 
ends.  See  that  the  lower  end  fits  accurately 
upon  the  plate  of  the  air-pump.  (It  is  well 
to  smear  the  plate  with  tallow  in  this  and 
similar  experiments.)  Place  the  hand  over 
the  other  end.  When  the  pump  is  worked, 
the  pressure  of  the  atmosphere  is  felt,  and 
the  hand  can  be  removed  only  by  a  consider- 
able effort.  The  appearance  of  the  palm  of  th?  hand  at  the 
end  of  this  experiment  is  due  to  the 
tension  of  the  air  within  tie  tissues 
of  the  hand. 


Experiment  62. — Perform  the  experi- 
ment described  in  §  179. 

Experiment  63. — Ov:>r  the  upper  end 
of  a  cylindrical  receiver,  tie  tightly  a  wet 
bladder  and  allow  it  to  dry.  Then  ex- 
haust the  air.  The  bladder  will  be 
forced  inward,  bursting  with  a  loud 
noise.  It  may  be  necessary  to  prick  a 


FIG.  65. 


FIG.  66 


§  187  AtR-P0Mt>S.  131 

pin-hole   through   the   bladder  after  the  receiver  has  been   ex 
hausted. 

Experiment  64.— Replace  the  bladder  with  a  piece  of  thin  india. 
rubber  cloth.      Exhaust  the  air.      The 
cloth  will  be  forced  inward  by  atmos- 
pheric pressure  and  nearly  cover   the 
inner  surface  of  the  receiver. 

NOTE.— Ths  hand-glass,  used  in  Ex- 
periment 61,  will  answer  for  the  two 
experiments  last  given,  by  placing  the 
small  end  upon  the  pump-plate. 

Experiment  65. — The  "fountain  in 
racuo  "  consists  of  a  glass  vessel,  through 
the  base  of  which  passes  a  tube  ter- 
minating in  a  jet  within,  and  provided 
with  a  stop-cock  and  screw  without. 
By  means  of  the  screw,  it  may  be  at-  -. 

tached  to  the  air-pump.      Remove  the 
air,  close  the  stop-cock,  place  the  lower  end  of  the  tube  in  water 
open  the  stop-stock  ;  a  beautiful  fountain  will  be 
produced  (Fig.  67). 

Experiment  66.— The  Magdeburg  hemispTieres 
are  made  of  metal  (Fig.  68).  They  are  hollow  and 
generally  three  or  four  inches  in  diameter.  The 
edges  being  greased  and  placed  together,  the  air  is 
exhausted  from  the  hollow  globe  through  a  tube 
provided  with  a  stop-cock  and  screw.  When  the 
air  has  been  pumped  out,  close  the  stop-cock  and 
remove  the  hemispheres  from  the  pump.  It  will 
l>e  found  that  a  considerable  force  is  necessary  to 
poll  the  hemispheres  asunder.  This  force  is 
equal  to  the  atmospheric  pressure  upon  the 
circular  area  inclosed  by  the  edges  of  the  hemispheres.  U 


PHILOSOPHY. 


§188 


ihis  area  be  ten  square  inches,  it  will  require  a  pull  of  150  pounds 
to  separate  the  hemispheres. 


Experiment  67.— Partly  fill  two  bottles  with 
water.  Connect  them  by  a  bent  tube  which  fits 
closely  into  the  mouth  of  one  and  loosely  into  the 
mouth  of  the  other.  Place  the  bottles  under  the 
receiver  and  exhaust  the  air.  Water  will  be  driven 
from  the  closely  stoppered  bottle  into  the  other. 
Readmit  air  to  the  receiver  and  the  water  thus 
driven  over  will  be  forced  back. 


FIG.  69. 


188.  The  Condenser. — TJie  condenser  is  an  in- 
strument for  compressing  a  large  amount  of  gas 
into  a  closed  vessel.  The  chief  difference  between 
it  and  the  air-pump  is  that  its  valves  open 
toward  the  receiver. 


189.    The   Lifting  -  pump. — 

The  lifting-pump  consists  of  a 
cylinder  or  barrel,  c,  a  piston,  p, 
two  valves,  v  v,  and  a  suction 
pipe,  s,  the  lower  end  of  which 
dips  below  the  surface  of  the 
liquid  to  be  raised.  The  arrange- 
ment is  essentially  the  same  as  in 
the  air-pump. 

As  the  piston  is  worked,  the  air 
below  it   is    gradually  removed. 
The  downward  pressure  on  the 
liquid  in  the  pipe  being  thus  re-  =2 
moved,  the  pressure  of  the  at-  0: 
mosphere,    exerted    upon    the 


FIG.  70. 


§191 


LIFTING  A\D   FORCE-PUMPS. 


133 


surface  of  the  liquid,  pushes  the  liquid  up 
through  the  suction  pipe  and  the  lower  valve 
into  the  barrel. 

When  the  piston  is  again  pressed  down,  the  lower 
valve  closes,  the  reaction  of  the  water  opens  the  piston 
valve  and  the  piston  sinks  below  the  surface  of  the  liquid 
in  the  barrel.  When  next  the  piston  is  raised,  it  lifts 
the  water  above  it  toward  the  spout  of  the  pump.  At 
the  same  time,  atmospheric  pressure  forces  more  liquid 
through  the  suction  pipe  into  the  barrel. 

190.  Practical  Points. — The  cistern  or  well  con- 
taining the  liquid  must  not  be  cut  off  from  atmospheric 
pressure,  i.  e.,  must  not  be  made  air-tight.  For  water 
pumps,  the  suction  pipe  must  not  be  more  than  34  feet 
high.  Owing  to  mechanical  imper- 
fections chiefly,  the  practical 
limit  of  the  water  pump  is 
about  28  vertical  feet. 


191.    The   Force -Pump. — In 

the  force-pump,  the  piston  is  often 
made  solid,  *.  e.,  without  any  valve. 
The  upper  valve  is  placed  in  a  dis- 
charge pipe,  d,  which  opens  from 
the  barrel  at  or  near  its  bottom. 

When  the  piston  is  raised,  water 
is  forced  into  the  barrel  by  atmos- 
pheric pressure.  When  the  piston 
is  forced  down,  the  suction  pipe  valve 
is  closed,  the  water  being  forced 


FIG.  7t 


134 


NATURAL  PHILOSOPHY. 


§192 


through  the  other  valve  into  the  discharge  pipe.  When 
the  piston  is  raised  again,  the  discharge  pipe  valve  is 
closed,  preventing  the  return  of  the  water  above  it,  while 
atmospheric  pressure  forces  more  water  from  below  into 
the  barrel. 

Sometimes  the  upper  valve  is  placed  in  the  piston,  as 
in  the  ordinary  lifting-pump,  the  discharge  pipe  opening 
from  the  upper  part  of  the  cylinder  which  is  closed  at 
the  top. 

192.  Direction  of  Valve  Openings. — In  the  case 
of  the  air-pump,  the  condenser,  the  lifting  and  force- 
pumps,  the  valves  open  in  the  direction  in  ivhich 
the  fluid  is  to  move. 


193.   The 


FIG.  72. 


Air-Chamber  of  a  Force-Pump. — 
Water  will  be  thrown  in  spurts  from 
a  pump  like  tli at  represented  in  Fig. 
71.  A  continuous  flow  is  secured 
by  connecting  the  discharge 
pipe  with  an  air-chamber. 

This  air-chamber,  c,  is  provided 
with  a  delivery  pipe,  b  or  s,  which 
reaches  below  the  surface  of  the  wa- 
ter in  the  air-chamber. 

When  water  is  forced  into  the  air- 
chamber,  it  covers  the  mouth  of  the 
delivery  pipe  and  compresses  the  air 
confined  in  the  chamber.  This  less- 


ening of  the  volume  of  the  air  causes  an  increased  ten- 


§  194  THE  SIPHON.  135 

sion  (§  179),  which  soon  becomes  sufficient  to  force  the 
water  through  the  delivery  pipe  in  a  continuous  stream. 
A  pump  may  have  both  delivery  pipes  but  one  of  them 
must  be  closed  by  a  cock,  as  shown  at  *, 

Experiment  68.— Set  a  pail  of  water  on  the  table.  Place  one 
end  of  a  piece  of  rubber  tubing  in  the 
water  and  let  the  other  end  hang  over 
the  edge  of  the  pail  reaching  below 
the  top  of  the  table.  Suck  some 
water  through  the  tube.  Water 
will  continue  to  flow  until  the  pail  is 
emptied  or  the  water  surface  falls 
below  the  end  of  the  tube  in  the  pail. 

194.    The    Siphon.  —  Tlie 
siphon    consists    of    a   bent  „ 

tube,    open    at    both    ends, 
having  one  arm  longer  than  the  other. 

It  is  used  to  transfer  liquids  from  a  higher  to  a  lower 
level,  especially  in  cases  where  they  are  to  be  removed 
without  disturbing  any  sediment  they  may  contain. 

The  siphon  may  be  first  filled  with  the  liquid  and  then 
placed  with  the  shorter  arm  in  the  vessel,  care  being  hud 
that  the  liquid  does  not  escape  from  the  tube  until  the 
opening,  C,  is  lower  than  m  n,  the  surface  of  the  liquid  ; 
or  it  may  be  first  placed  in  position  and  the  air  removed 
by  suction  at  the  lower  end. 

The  pressure  of  the  atmosphere  will  force  the  liquid 
up  the  shorter  arm  and  fill  the  tube.  In  either  case,  a 
constant  stream  of  the  liquid  will  flow  from  the  vessel 


136 


NATURAL  PHILOSOPHY. 


§195 


until  the  surface  line,  m  n,  is  brought  as  low  as  the 
opening  in  the  shorter  arm  or,  if  the  liquid  be  received 
in  another  vessel,  until  the  level  is  the  same  in  the  two 


(a.)  The  action  of  the  siphon  is  explained  in  the  author's  larger 
work.  If  the  liquid  to  be  transferred  is  water,  the  height,  a  b, 
must  be  less  than  thirty-four  feet. 

195.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


PUMPS.. 


DEFINITION. 

CONSTRUCTION. 

AIR 

LIMITATIONS. 

EXPERIMENTS. 

CONDENSER. 

WATER.  • 

!  LIFTING. 
FORCE. 

Valve  Openings. 


THE    SIPHON. 


§195 


EXERCISES. 


137 


EXERCISES. 

1.  What  is  the  total  atmospheric  pressure  (in  pounds)  upon  the 
surface  of  a  wooden  cu  be  one  inch  on  each  edge  ? 

2.  If  mercury  is  13.6  times  as  heavy  as  water,  and  a  given 
pump  will  lift  water  to  the  height  of  28  feet,  how  high  will  the 
same  pump,  other  conditions  being  similar,  lift  the  mercury  ? 

3.  How  can  you  arrange  a  single  suction  or  lifting-pump  to 
raise  water  from  the  bottom  of  a  well  50  feet  deep  ? 

4.  Construct  the  apparatus  shown 
in  Fig.  74,   filling  each  of  the  three 
bottles  half  full  of  water.      Blow  in 
the  tube,/,  until  a  jet  is  formed  at  n. 
Explain  the  continued  action   of  the 
apparatus. 

Be  sure  that  all  joints  made  by  the 
corks  of  the  three  bottles  are  air-tight. 
For  directions  in  working  glass  tubing, 
see  Chemistry,  Appendix  4. 

5.  How  many  valves  are  there  in 
a  force-pump?   Where  are  they  placed 
and  in  what  direction  do  they  open  ? 

6.  What  is  the   thing   sometimes 
erroneously  called  "the  force  of  suc- 
tion"? FIG.  74. 

7.  The  volume  of  a  given  quantity  of  r,  gas  or  vapor  will  be 
inversely  proportional  to  the  pressure  exerted  upon  it.     A  cubic 
foot  of  steam  (measured  under  a  pressure  of  one  atmosphere)  will 
have  what  volume  under  a  pressure  of  8  atmospheres  ? 

An*.  216  cubic  inches. 


138  NATURAL  PHILOSOPHY.  §  195 


REVIEW     QUESTIONS. 

1.  (a.)  When  an  inverted  bottle  is  held  under  water,  why  does 
not  the  water  fill  it  ?    (b.)  Why  is  it  that  any  water  enters  the 
bottle  ? 

2.  (a.)  When  the  sails  of  a  ship  are  taken  down,  why  does  not 
the  boat  suddenly  stop  ?    (b.)  What  finally  stops  the  ship  ? 

3.  Why  does  not  a  stone  move  in  a  straight  line  when  thrown 
horizontally  ? 

4.  What  is  the  difference  between  the  words  "  vertical "  and 
perpendicular  "  ? 

5.  When  a  bullet  is  flattened  by  the  target,  what  "  law "  is 
thereby  illustrated  ? 

6.  Why  does  a  freely  falling  body  increase  in  velocity  ? 

7.  What  has  become  of  the  energy  expended  centuries  ago  in 

building  the  still  remaining 
parts  of  the  Egyptian  pyra- 
mids? 

8.  What  kind  of  a  lever  is 
represented  in  Fig.  75?     In- 
dicate the  positions  for  P,  W 
and  F. 

9.  What  is  always  the  dividend  in  problems  in  specific  gravity  ? 

10.  If  a  pendulum,  having  a  certain  length  and  a  weight  of  2 
ounces,  vibrates  once  a  second,  how  often  will  a  pendulum  having 
the  same  length  and  a  weight  of  6  ounces  vibrate  ? 

11.  State  the  law  of  weight. 

12.  State,  in  ordinary  language,  the  meaning  of  the  formula, 

»  =  \gt*. 

18.  What  is  the  difference  between  Kinetic  and  Potential  En- 
ergy? 


§195 


REVIEW   QUESTIONS. 


139 


FIG.  76. 

14.  A  stratum  of  sand  or  gravel  through  which  water  can 
easily  work  its  way,  comes  to  the  surface  of  the  ground  at  a,  Fig. 
76.  This  stratum  is  inclosed  between  two  curved  strata  imper- 
vious to  water.  When  a  hole  is  bored  at  c  until  it  strikes  the 
stratum,  a,  an  artesian  well  is  formed.  Explain  the  biiion  of  the 
artesian  well 


CHAPTER    VI. 

ELECTRICITY    AND    MAGNETISM. 


SECTION     I. 
GENERAL    VIEW. 

196.   Importance  of  the  Subject. — We  are  now 

to  begin  the  study  of  a  class  of  phenomena  of  intense 
interest  and  almost  unlimited  practical  importance. 
Every  pupil  has  seen  the  lightning  flashing  in  the  stormy 
sky  and  wondered  at  the  cause  of  the  terrible,  yet  beauti- 
ful, display.  Later  in  life,  he  was  told  that  lightning  is 
electricity.  He  has  seen  or  heard  of  trees  and  houses 
shattered  by  the  lightning  stroke  and  may  now  remem- 
ber that  if  electricity  can  do  this  work,  it  must 
be  a  form  of  energy. 

He  is  told  that  the  telegraph  is  electricity  under  con- 
trol and  working  at  the  will  of  man.  When  he  reads 
^he  daily  paper  and  learns  of  events  that  took  place  only 
a  few  hours  before  in  Europe,  Asia  or  Africa  as  well  as 
in  every  part  of  his  own  country,  he  must,  it  seems, 
think  of  the  wondrous  speed  of  this  form  of  energy  as  it 
courses  under  the  waters  of  the  sea  and  over  the  valleys, 
plains  and  mountains  of  the  land.  With  the  aid  of  ttig 


§  Ip7  GENERAL    VIEW.  141 

telephone,  he  talks  with  friends  and  recognizes  their 
voices  though  they  be  miles  away.  He  is  a  fit  subject 
for  pity  if  he  has  no  desire  to  know  how  these  things 
are  done. 

He  hears  of  the  mysterious  power  that  points  the 
needle  of  the  mariner's  compass  to  the  north  and  guides 
the  ship  across  the  trackless  waters.  He  is  told  that 
this  power  is  called  magnetism,  that  it  is  closely  asso- 
ciated with  electricity  and  is  frequently  produced  by  it. 
He  sees  the  electric  light  and  is  surprised  to  find  that, 
in  turn,  magnetism  produces  electricity.  He  talks  with 
the  merchant  or  manufacturer  about  these  and  other 
things,  that  were  lately  wonders  and  conveniences  but 
have  now  become  common-place  necessities  of  business 
life,  and  finds  that  the  so-called  "practical "  man  of  the 
world  is  forced  to  acknowledge  his  great  and  ever  in- 
creasing obligation  to  modern  science.  He  probably 
gets  the  idea  that  it  iv ill  pay  for  him  to  learn  more  about 
these  things. 

197.  Simple  Apparatus. — Provide  two  stout  sticks 
of  sealing-wax  and  one  or  two  pieces  of  flannel  folded  into 
pads  about  20  centimeters  (8  inches)  square ;  two  glass 
rods  or  stout  tubes  closed  at  one  end,  rs—  Q 

30  or  40  centimeters  in  length  and 
about  2  centimeters  in  diameter  (long  IG' 

"ignition  tubes"  will  answer)  and  one  or  two  silk  pads 
about  20  centimeters  square,  the  pads  being  three  or 
four  layers  thick  ;  a  few  pith  balls  about  1  centimeter  in 
diameter  (whittle  them  nearly  round  and  finish  by  roll- 
ing them  between  the  palms  of  the  hands);  a  silk  ribbon 


142  NATURAL  PHILOSOPHY.  §  IQ7 

about  an  inch  wide  and  a  foot  long ;  a  balanced  straw 
about  a  foot  long,  represented  in  Fig.  77.  The  ends  of 
the  straw  carry  two  small  discs  of  paper  (bright  colors 
preferable)  fastened  on  by  sealing-wax.  The  cap  at  the 
middle  of  the  straw  is  a  short  piece  of  straw  fastened  by 
sealing-wax.  This  is  supported  upon  the  point  of  a 
sewing- needle,  the  other  end  of  which  is  stuck  upright 
into  the  cork  of  a  small  glass  vial.  From  the  ceiling  or 
other  convenient  support,  suspend  one  of  the  pith  balls 
by  a  fine  silk  thread. 

(a.)  The  efficiency  of  the  silk  pad  above  mentioned  may  be 
increased  by  smearing  one  side  with  lard  and  applying  an  amal 
gam,  made  of  one  weight  of  tin,  two  of  zinc  and  six  of  mercury. 
The  amalgam  which  may  be  scraped  from  bits  of  a  broken  looking- 
glass  answers  the  purpose  admirably.  A  piece  of  cat-skin  or  other 
fur  may  be  used  instead  of  the  flannel  pads.  See  that  the  sealing- 
wax  and  glass  rods,  the  flannel  and  silk  pads  are  perfectly  dry. 
Have  them  quite  warm,  that  they  may  not  condense  moisture  from 
the  atmosphere. 

Experiment  69. — Draw  the  silk  ribbon  between  two  layers  of 
the  warm  flannel  pad  with  considerable  friction.  Hold  it  near  the 
wall  of  the  room.  The  ribbon  will  be  drawn  to  the  wall  and 
held  there  for  some  time. 

Place  a  sheet  of  paper  on  a  warm  board  and  briskly  rub  it  with 
india-rubber.  Hold  it  near  the  wall  as  you  did  the  ribbon. 

Experiment  70. — Briskly  rub  the  sealing-wax  with  the  flannel 
and  bring  the  wax  near  the  suspended  pith  ball.  The  ball  will  be 
drawn  to  the  wax.  Bring  the  wax  near  one  end  of  the  balanced 
straw  ;  it  may  be  made  to  follow  the  wax  round  and  round.  Bring 
it  near  small  scraps  of  paper,  shreds  of  cotton  and  silk,  feathers 
and  gold  leaf,  bran  and  sawdust,  and  other  light  bodies;  they  are 
attracted  to  the  wax.  (Fig.  78.) 


§197 


GENERAL 


143 


Experiment  71. — 
Repeat  all  of  these  ex- 
periments with  a  glass 
rod  which  has  been 
rubbed  with  the  silk 
pad. 

Experiment  72. — 

Make  a  light  paper 
hoop  or  an  empty  egg- 
shell roll  after  your 
rod. 


FIG.  78. 


Experiment  73.— Place  an  egg  hi  a  wine-glass  or  an  egg-cup. 
Upon  the  egg  balance  a  yard-stick  or  a  common  lath.  The  end  of 
the  stick  may  be  made  to  follow 
the  rubbed  rod  round  and  round. 
Place  the  blackboard  pointer  or  other 
stick  in  a  wire  loop  or  stiff  paper 
stirrup  suspended  by  a  stout  silk 
thread  or  narrow  silk  ribbon.  It 
may  be  made  to  imitate  the  actions 
of  the  balanced  straw  or  lath. 


FIG.  79. 


Experiment  74.  —  Suspend  the 
rubbed  sealing-wax  or  glass  rod  as 
you  did  the  blackboard  pointer  in 
the  last  experiment.  Hold  your 
hand  near  the  end  of  the  rod.  It  will  turn  round  and  approach 
your  hand. 

NOTE. — The  pupil  may  be  ingenious  enough  to  invent  new 
experiments  for  himself  and  the  class.  The  ability  to  invent  is 
often  very  valuable  and  may  be  acquired  early  in  life.  Most 
of  the  great  inventors  began  making  experiments  when  mere 
children. 


144 


NATURAL  PHILOSOPHY. 


198.  Electric  Attraction.— The  attractions  man- 
ifested in  these  ex- 
periments were  due 
to  electricity  that 
was  developed  by 
friction. 


Experiment  75.— Bring 
the  rubbed  sealing-wax  or 
glass  rod  near  the  pith  ball 
again.  It  will  attract  the 
ball  as  in  Experiment  70. 
Allow  the  ball  to  touch  the 
rod  and  notice  that  in  a  mo- 
ment the  ball  is  thrown  off. 
If  the  ball  be  pursued  with 
the  rod,  it  will  be  found 
that  the  rod  which  at- 
tracted it  a  moment  ago, 
now  repels  it.  Evidently 


FIG.  80. 


the  ball  has  acquired  a  new  property. 


Experiment  76.  —Touch  the  ball  with  the  finger.  It  seeks  the 
rubbed  rod,  touches  the  rod,  flies  from  the  rod.  Repeat  the  ex- 
periments with  the  sealing-wax  after  it  has  been  rubbed  with 
flannel. 

Experiment  77. — Rub  the  glass  rod  with  silk  and  bring  it  over 
the  small  scraps  of  paper,  as  in  Experiment  71.  Notice  that  after 
the  attraction  the  paper  bits  do  not  merely  fall  down,  they  are 
thrown  down. 

199.  Electric  Repulsion. — The  repulsions  mani- 
fested in  these  experiments  were  due  to  elec- 
tricity that  was  developed  by  friction.  Such 


§199 


GENERAL    V7EW. 


145 


electricity    is    called    frictional    or    static    elec- 
tricity, 

The  glass  or  wax  is  said  to  be 
electrified  by  friction.  The 
ball,  after  obtaining  its  new 
property  of  repulsion  by  com- 
ing into  contact  with  the  glass 
or  wax  is  said  to  be  electrified 
by  conduction.  The  sus- 
pended pith  ball  is  called  an 
electric  pendulum. 

Experiment  78.— Prepare  a  bat- 
tery solution  according  to  the  recipe 
given  in  §  258,  6,  using  only  half  the 
quantity  of  each  substance  as  therein 
directed.  While  the  solution  is  cool- 
ing, provide  a  piece  of  sheet  copper 
and  one  of  sheet  zinc,  each  about  10  centimeters  (4  inches)  long 
and  4  centimeters  (1£  inches)  wide.  To  one  end  of  each  strip, 
solder  (see  Appendix)  or  otherwise  fasten  a  piece  of  copper  wire 
about  15  centimeters  (6  inches)  long  and  1  millimeter  (TV  to  •£% 
inch)  thick.  Place  the  zinc  strip  in  a  common  tumbler  about 
three-fourths  full  of  the  battery  solution.  Notice  the  minute 
bubbles  that  break  away  from  the  surface  of  the  zinc  and  rise  to 
the  surface  of  the  liquid.  These  are  bubbles  of  hydrogen,  a  com- 
bustible gas.  The  formation  of  the  gas  is  due  to  chemical 
action  between  the  zinc  and  the  liquid. 

Experiment  79. — Take  the  zinc  from  the  tumbler  and,  while 
it  is  yet  wet,  rub  a  few  drops  of  mercury  (quicksilver)  over  its 
surface  until  it  has  a  brilliant,  silver-like  appearance.     Replace  the  ' 
zinc,  thus  amalgamated,  in  the  solution  and  notice  that  no  bub- 
bles are  given  off. 


FIG.  81. 


146 


NATURAL   PHILOSOPHY. 


§200 


Experiment  80. — Place  the  copper  strip  in  the  liquid,  taking 
care  that  it  or  its  wire  does  not  touch  the  zinc  or  its  wire.     No 
bubbles  appear  on  either  the  zinc  or  the  copper.    It  may  he 
convenient  to  place  a  narrow  glass  strip  be- 
tween the  ends  of  the  metal  strips  in  the  tum- 
bler to  keep  them  apart. 


Experiment  81. — Bring  the  upper  ends  of 
the  strips  together,  as  shown  in  Fig.  82,  or,  still 
better,  join  the  two  wires,  as  shown  in  Fig.  102. 
being  sure  that  the  wires  are  clean  and  bright 
where  they  are  united.  Notice  the  formation 
of  bubbles  on  the  surface  of  the  copper, 
where  none  previously  appeared.  (§  248.) 


FIG.  83. 


20O.  Suspicion. — It  certainly  seems  that  the  connect- 
ing wire  is  an  important  part  of  the  apparatus  as  now 
arranged  and  we  are  led  to  suspect  that  something  un- 
usual is  taking  place  in  the  wire  itself.  It  is  evident 
that  we  have  a  complete  "  circuit "  through  the  liquid, 
the  metal  strip  and  the  wire. 

Experiment  82. — Untwist  the  wires  or, in  other  words,  "break 
the  circuit."  Connect  the  copper  wires  with  a  short  piece  of  very 
f.ne  iron  wire.  The  connections  should  be  made  so  that  the  cir- 
cuit shall  include  about  2  centimeters  (f  inch)  of  iron  wire.  The 
ron  will  become  hot  enough  to  burn  the  fingers  or  to  ignite  a 
small  quantity  of  gun  cotton  twisted  around  it. 


Experiment  83.— If  one  of  the 
copper  wires  be  twisted  around 
one  end  of  a  small  file  and  the 
other  wire  be  drawn  along  its 
rough  surface,  a  series  of  minute 
sparks  will  be  produced  as  the 
circuit  is  rapidly  made  and  broken. 


FIG. 


§201 


GENERAL    VIEW. 


147 


Experiment  84.— Place  the  cell  so  that  the  joined  wires  sliaT. 
run  north  and  south,  passing  directly  over  the  needle  of  a  small 
compass  ($  295,  ft.)  and  near  to  it.  The  needle  will  instantly  turn 
as  though  it  were  trying  to  place  itself  at  right  angles  to  the  wire 
Break  the  circuit  and  the  needle  will  swing  hack  to  its  north  and 
south  position. 


FIG.  84. 

201.  Certainty. — We  now  feel  sure  that  something 
unusual  is  taking  place  in  the  wire  of  our  complete  cii> 
cuit  for  we  have  seen  the  wire  become  hot,  explode  gun- 
cotton,  yield  sparks  and  exert  a  very  mysterious  influence 
upon  the  magnetic  needle.  As  a  matter  of  fact,  we  now 
have  a  current  of  electricity  flowing  through  a  galvanic 
cell. 

Electricity  thus  produced  by  chemical  action 
is  called  galvanic  or  voltaic  electricity.  It  is  one 
form  of  current  electricity. 

Experiment  85.  —  Wrap  a  piece  of  writing  paper  around  a 
large  iron  nail  leaving  the  ends  of  the  nail  bare.  Wind  fifteen 
or  twenty  turns  of  stout  copper  wire  around  this  paper  wrapper, 
taking  care  that  the  coils  of  the  wire  spiral  do  not  touch  each 
other  or  the  Iron,  It  is  well  to  use  cotton  covered  or  "  insulated  * 


148  NATURAL  PHILOSOPHY.  §  202 

wire.  Connect  the  two  ends  of  the  wire  spiral  with  the  two  wires 
of  the  galvanic  cell  or,  in  other  words,  put  the  spiral  into  the  cir- 
cuit. Dip  the  end  of  the  nail  into  iron  filings.  Some  of  the 
filings  will  cling  to  the  nail  in  a  remarkable  manner.  Upon 
breaking  the  circuit,  the  nail  instantly  loses  its  newly  acquired 
power  and  drops  the  iron  filings. 

If  the  experiment  does  not  work  satisfactorily,  look  carefully  to 
all  the  connections  of  the  circuit,  see  that  the  ends  of  the  wires 
are  clean  and  bright  and  that  they  are  twisted  together  firmly.  It 
may  even  be  necessary  to  wash  the  plates,  rub  more  mercury  on 
the  zinc  and  provide  a  fresh  battery  solution. 

202.  Temporary  Magnets.  —  You  ha -e  probably 
satisfied  yourself  that  the  nail  has  the  power  A  attract- 
ing iron   filings  while  the  electric  current  is  flow- 
ing through  the  wire. 

You  have  made  an  electro-magnet  and  its 
power  of  attracting  iron  is  called  magnetism. 

Satisfy  yourself,  by  trial,  that  the  nail  loses  its  mag- 
netism as  soon  as  the  circuit  is  broken  or  the  current 
ceases  to  flow  around  it  and  remember  that  your  electro- 
magnet is  a  temporary  magnet. 

Experiment  86.  —While  the  nail  is  magnetized,  draw  a  sewinjr- 
needle  four  or  five  times  from  eye  to  point  across  one  end  of  the 
electro-magnet.  Dip  the  needle  into  iron  filings ;  some  of  them 
will  cling  to  each  end  of  it. 

203.  Permanent  Magnets. — When  steel  is  treated 
as  in  the  last  experiment,  it  becomes  permanently  mag- 
netized. 

Experiment  87.— Cut  a  thin  slice  from  the  end  of  a  vial  cork 
and,  with  its  aid,  float  your  magnetized  needle  upon  the  surface  of 


§  207  GENERAL    VIEW.  149 

a  bowl  or  saucer  of  water.  The  needle  comes  to  rest  in  a 
north  and  south  position.  Turn  it  from  its  chosen  position  and 
notice  that  after  each  displacement  it  resumes  the  same  position 
and  that  the  same  end  of  the  needle  always  points  to  the 
north. 

204.  A  Simple  Compass.— t^  small  magnetized 
steel  bar  freely  suspended,  is  called  a  compass. 

The  one  that  you  have  made  may  be  less  convenient 
but  is  as  reliable  as  the  compass  of  the  mariner  or  the 
surveyor. 

205.  Artificial  Magnets. — The  electro-nrugnet  and 
the  permanent  magnet  that  you  made  are,  of  course, 
artificial   magnets.      There  is  a   natural   magnet 
known  as  lodestone. 

206.  Other    Forms   of   Current    Electricity. — 

Electric  currents  may  be  generated  by  the  action  of 
other  currents  of  electricity  or  by  the  action  of  magnets. 
Electricity  thus  developed  is  called  induced  electricity. 
A  current  of  electricity  that  is  generated  by  heating 
the  junction  of  two  metals  that  form  part  or  all  of  a  cir- 
cuit is  called  thermo-electricity. 

207.  The    Different    Forms  of  Electricity  are 
Identical.— So  far  as  experiment  can  show,  one  form  of 
electricity  may  have  a  particular  property  in  greater 
degree  than  some  other  form  but  all  are  identical,  each 
having  all  the  properties  of  any  of  the  others. 


150 


NATURAL   PHILOSOPHY. 


§208 


208.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


All  Forms  of  Electricity   are  Iden- 
tical in  Nature. 


s 

W  M 

H        P- 


$210  ELECTRICITY.  151 


SECTION     II. 

FRICTIONAL    ELECTRICITY    OR    ELECTRIC 
CHARGES. 

209.  The  Nature  of  Electricity.  —  But  little  is 
known  concerning  the  real  nature  of  electricity.     It  is 
easier  to  tell  what  electricity  can  do  than  to  tell  what 
it  is. 

The  majority  of  modern  physicists  consider  that  elec- 
tricity is  a  form  of  energy  producing  peculiar 
phenomena;  that  it  may  be  converted  into  other 
forms  of  energy  and  that  all  other  forms  of 
energy  may  be  converted  into  it. 

Several  theories  have  been  advanced  to  account  for 
electrical  phenomena  but  none  of  them  is  satisfactory. 

210.  Electric  Manifestations.— Electricity  may 
reveal  itself  as  a  charge  or  as  a  current. 

By  means  of  friction,  the  glass  rod  or  the  sealing  wax 
(§§  198,  199)  acquired  an  electrical  charge  and,  conse- 
quently, the  power  of  attracting  and  repelling  light 
bodies:  by  means  of  chemical  action,  the  galvanic  cell 
generated  electricity  that  manifested  itself  as  a  current. 
In  this  section,  ^ve  shall  consider  electricity  that  appears 
as  a  charge,  i.  e.,  static  electricity. 

Experiment  88.— Prepare  two  electric  pendulums.  Bring  the 
electrified  glass  rod  near  the  pith  ball  of  one  ,  after  contact,  the 


NATURAL  PHILOSOPHY. 


§210 


ball  will  be  repelled  by  the  glass.  Bring  the  electrified  sealing. 
wax  near  the  second  pith  ball ;  after  contact  it  will  be  repelled  by 
the  wax.  Satisfy  yourself  that  the  electrified  glass  will  repel  the 
first ;  that  the  electrified  sealing-wax  will  repel  the  second.  Let 
the  glass  rod  and  the  sealing  wax  change  hands.  The  first  ball 
was  repelled  by  the  glass ;  it  will  be  attracted  by  the  sealing- 
wax.  The  second  ball  was  repelled  by  the  sealing-wax  ;  it  will  be 
attracted  by  the  glass. 

Experiment  89.— Suspend  two  pith  balls,  as  shown  in  Fig  85, 

and  touch  them  with  a 
rubbed  glass  rod.  Instead 
of  continuing  to  hang  side 
by  side,  they  repel  each 
other  and  fly  apart.  If 
the  electrified  glass  rod 
be  held  near  them,  they 
separate  still  further.  If 
the  electrified  sealing- 
wax,  instead  of  the  glass, 
be  held  near  them  they 
will  fall  nearer  together. 
If  the  rubbed  glass  rod  be 
suspended,  as  shown  ia 
Fig.  79,  it  will  be  repelled 
by  another  rubbed  glass  rod  but  attracted  by  rubbed  sealing-wax. 

211.  Two  Kinds  of  Electricity.— The  electricity 
developed  on  glass  "is  different  in  kind  from  that 
developed  on  sealing-wax. 

They  exhibit  opposite  forces  to  a  third  electrified  body, 
each  attracting  what  the  other  repels.  The  self-repul- 
sion of  the  parts  of  an  electrified  body  may  be  beautifully 
illustrated  by  electrifying  a  soap-bubble  which  will  thu& 
be  made  to  expand.. 


FIG.  85. 


§  215  FBICTIONAL  ELECTRICITY.  153 

212.  Only  Two  Kinds  of  Electricity.—  All  elec- 
trified  bodies    act   like    either   the  glass  or   the 
sealing-wax. 

213.  The  Two  Electricities  Named.— The  elec- 
tricity developed  on  glass  by  rubbing  it  with  silk 
is  called  positive  or  +  . 

The  electricity  developed  on  sealing-wax  by 
rubbing  it  with  flannel  is  called  negative  or  — . 

The  terms  vitreous  and  resinous  respectively  were 
formerly  used. 

214.  The  Law  of  Electrostatics.— The  most  im- 
portant electrostatic  law  may  be  stated  thus  : 

Electric  charges  of  like  signs  repel  each  other; 
electric  charges  of  opposite  signs  attract  each 
other. 

215.  Electroscopes. — An   instrument   used    to 
detect  the  presence  of  electricity,  or  to  determine 
its  kind,  is  called  an  electroscope. 

The  electric  pendulum  (§  199)  is  a  common  form  of 
the  electroscope.  Two  strips  of  the  thinnest  tissue  paper 
hanging  side  by  side  constitute  a  simple  electroscope. 
It  is  well  to  prepare  the  paper  beforehand  by  soaking  in 
a  strong  solution  of  salt  in  water  and  drying. 

The  balanced  straw  (Fig.  77)  or,  better  yet,  two  gilt 
pith  balls  connected  by  a  light'  needle  of  glass  or  sealing- 
wax  balanced  horizontally  on  a  vertical  pivot  or  a  goose- 
quill  balanced  on  the  point  of  a  sewing-needle,  makes  $ 
convenient  electroscope. 


J54 


NATURAL  PHILOSOPHY. 


§215 


The  gold  leaf  electroscope  is  represented  in  Fig.  86. 
A  metallic  rod,  which 
passes  through  the  cork 
of  a  glass  vessel,  termi- 
nates below  in  two  narrow 
strips  of  gold  leaf  and 
above  in  a  metallic  knob 
or  plate.  The  object  of 
the  vessel  is  to  protect  the 
leaves  from  disturbance 
by  air  currents.  The 
upper  part  of  the  glass  is 
often  coated  with  a  solu- 
tion of  sealing-wax  or 
shellac  in  alcohol,  to 
lessen  the  deposition  of 
moisture  from  the  atmosphere.  This  instrument  may 
be  made  by  the  pupil  and,  when  well  made,  is  very 
delicate. 

Experiment  90.  —  From  a  horizontal  glass  rod  or  tightly- 
stretched  silk  cord,  suspend  a  fine  copper  wire,  a  linen  thread  and 
two  silk  threads,  each  at  least  a  yard  long.  To  the  lower  end  of 
each,  attach  a  metal  weight  of  any  kind.  Place  the  weight  sup- 
ported by  the  wire  upon  the  plate  of  the  gold  leaf  electroscope. 
Bring  the  electrified  glass  rod  near  the  upper  end  of  the  wire ;  the 
gold  leaves  instantly  diverge.  Repeat  the  experiment  with  the 
linen  thread :  in  a  little  while  the  leaves  diverge.  Repeat  the 
experiment  with  the  dry  silk  -thread ;  the  leaves  do  not  diverge 
at  all.  Rub  the  rod  upon  the  upper  end  of  the  silk  thread  ;  no 
divergence  yet  appears.  Wet  the  second  silk  cord  thoroughly 
and,  with  it,  repeat  the  experiment ;  the  leaves  then  diverge 


PIG.  86 


§  217  FRICTION AL  ELECTRICITY.  155 

216.  Conductors.— Such  experiments  clearly  show 
that  some  substances  transmit  electricity  readily 
and  that  others  do  not. 

Those  that  offer  little  resistance  to  the  passage 
of  electricity  are  called  conductors;  those  that 
offer  great  resistance  are  called  non-conductors 
or  insulators. 

A  conductor  supported  by  a  non-conductor  is  said  to 
be  insulated. 

(a.)  In  the  following  table,  the  substances  named  are  arranged 
in  the  order  of  their  conductivity  : 


Conductors. 

1.  Metals. 

2.  Charcoal. 

3.  Graphite. 

4.  Acids. 


5.  Salt  wa'ir. 

6.  Freshwater. 

7.  Vegetables. 

8.  Animals. 

9.  Linen. 


10.  Cotton. 

11.  Dry  wood. 

12.  Paper. 

13.  Silk. 


;14.  India  rubber.        Insulators. 


15.  Porcelain. 

16.  Glass. 

17.  Sealing-wax. 

18.  Vulcanite. 


The  fact  that  a  conductor  in  the  air  may  be  insulated,  shows 
that  air  is  a  non  conductor.  Dry  air  is  a  very  good  insulator,  but 
moist  air  is  a  fairly  good  conductor  for  electricity  of  high  potential. 
All  experiments  in  frictional  electricity  should,  therefore,  be 
performed  in  clear,  cold  weather  when  the  atmosphere  is  dry, 
for  a  moist  atmosphere  renders  insulation  for  a  considerable  length 
of  time  impossible. 

Experiment  91. — Support  a  yard-stick  or  common  lath  upon 
a  glass  tumbler.  Bring  the  glass  rod,  electrified  by  rubbing  it  with 
silk,  to  one  end  of  the  stick  and  hold  some  small  pieces  of  paper 
under  the  other  end  of  the  stick.  The  paper  will  be  attracted  and 
repelled  by  the  stick  as  it  previously  was  by  the  glass  itself.  The 
electricity  passed  along  the  stick  from  end  to  end. 

217.  Tension.— Electricity  exists  under  widely  dif- 
ferent conditions  with  respect  to  its  ability  to  force  its 
way  through  a  poor  copductor  or  to  leap  across  a  gap. 


156  NATURAL  PHILOSOPHY.  §  217 

The  electricity  developed  in  a  galvanic  cell  will  not 
pass  through  even  a  very  thin  piece  of  dry  wood ;  the 
electricity  developed  by  rubbing  the  glass  rod  will  pass 
through  several  feet  of  dry  wood. 

It  would  require  a  battery  of  many  cells  to  force  a 
current  across  a  gap  of  y^  of  an  inch.  It  is  not  diffi- 
cult to  force  frictional  electricity  across  a  gap  of  several 
inches  while  we  all  know  that,  in  the  case  of  lightning, 
electricity  leaps  across  a  gap  of  many  hundred  feet. 

In  the  one  case,  the  electricity  is  said  to  be  of  low 
potential;  in  the  other  case,  it  is  said  to  be  of  high  po- 
tential. We  are  now  dealing  with  electricity  of  high 
potential. 

The  terms,  "low  tension"  and  "high  tension"  are 
often  used  in  the  same  sense. 

218.  Potential.  —  The  term  electrical  potential  (or 
simply  potential)  has  reference  to  the  electrical  condi- 
tion of  a  body  or  to  its  degree  of  electrification.     If  the 
potential  of  A  be  higher  than  that  of  B,  and  the  two 
bodies  be  connected  by  a  good  conductor,  an  electric 
current  will  flow  from  A  to  B  until   the  poten- 
tials are  alike. 

Difference  of  potential  is  somewhat  analogous  to  differ- 
ence of  liquid  level  and  gives  rise  to  electromotive  force. 

219.  Electromotive  Force.  —  Electromotive  force 
(often  written  E.  M.  F.)  is  the  mysterious  power  which 
causes  electricity  to  move  from  one  point  to  another. 
It  is  somewhat  analogous  to  hydrostatic  pressure.    Wbcr- 


S  220  FRICTION AL   ELECTRICITY.  151 

ever  there  is  difference  of  potential,  there  is  E.  M.  F.t 
but  the  terms  are  not  synonymous. 

The  unit  of  electromotive  force  is  called  a  volt. 

A  volt  is  a  little  less  than  the  E.  M.  F.  of  a  Danieli 
dell  (§  260). 

220.  Resistance.— Every  electric  circuit  offers  a  re- 
sistance to  the  passage  of  the  current.  This  resistance 
will,  of  course,  depend  largely  upon  the  conductivity  of 
the  material  used  for  the  circuit. 

With  a  given  material  for  the  conductor,  the 
resistance  varies  directly  as  the  length  and  in- 
versely as  the  weight  of  a  given  length. 

If  one  conductor  is  twice  as  long  as  another  made  of 
the  same  kind  of  wire,  the  resistance  of  the  longer  will 
be  twice  as  great  as  that  of  the  shorter. 

If  one  conductor  be  twice  the  diameter  of  another  made 
of  the  same  length  and  material,  the  weight  per  foot  or 
yard  will  be  (2Z  =)  four  times  as  great  and  the  resistance 
of  the  first  will  be  one-fourth  as  great  as  that  of  the 
second. 

If  they  be  made  of  the  same  material  and  length,  one 
weighing  twice  as  much  per  foot  as  the  latter,  the  resist- 
ance of  the  former  will  be  half  as  great  as  that  of  the 
latter. 

The  unit  of  resistance  is  called  an  ohm. 

A  galvanized  iron  (telegraph)  wire,  4  millimeters  in 
diameter  and  100  meters  long,  or  a  pure  copper  wire, 
1  millimeter  in  diameter  and  48  meters  long,  has  a  re- 
sistance of  about  one  ohm. 

A  megohm  is  a  million  ohms. 


158  XAWRAL   P&ILOSOPHT.  §  221 

221.  Charging  by  Contact. — If  an  insulated,  un- 
electrified  conductor  be  brought  into  contact  with   a 
similar  conductor  that  is  electrified,  or  near  enough  to 
it  for  the  passage  of  an  electric  spark,  electricity  will 
pass  from  the  latter  to  the  former  until  the  two  con- 
ductors are  equally  charged  with  the  same  kind  of  elec- 
tricity.    The  former  is  said  to  be  charged  by  con- 
duction. 

222.  Induction. — Actual  contact  with  an  electrified 
body  is  not  necessary  for  the  manifestation  of  electric 

action  in  an  unelectri- 

q.       -  fied  body.     When  an 

ft      B     |  \\J\c  electrified  body,  C,  is 

oo  •      vb\  brought  near  an  insu- 

.^     % 

*^  with  electric  pendu- 
lums, as  shown  in  the 
figure,  the  latter  shows 
electric  action.  The 
electricity  of  C  rebels 

one  kind  of  electricity  in  B  and  attracts  the  other,  thus 
separating  them.  The  second  body,  B,  is  then  said  to 
be  polarized. 

The  two  kinds  of  electricity  in  J9,  each  of  which  a 
moment  ago  rendered  the  other  powerless,  are  still  there 
but  they  have  been  separated  and  each  clothed  with  its 
proper  power.  This  effect  is  due  to  the  action  of  the 
electrified  body,  C,  which  is  said  to  produce  electric 


^Pj£\  lated  unelectrified  con- 

m^\  ductor,    B,    provided 


g  223  PRICTIONAL   ELECTRICITY.  159 

separation  by  induction.  When  C  is  removed,  the  sep- 
arated electricities  of  B  again  mingle  and  neutralize 
each  other. 

(a.)  Conductors  for  the  purposes  of  this  and  similar  experiments 
may  be  made  of  wood  covered  with  tin-foil,  gold  leaf  or  Dutch 
leaf.  They  may  be  insulated  by  fastening  them  on  top  of  long 
necked  bottles  or  sticks  of  sealing-wax  or  by  suspending  them 
by  silk  threads. 

(6.)  Prick  a  pin  hole  in  each  end  of  a  hen's  egg  and  blow  on 
the  white  and  the  yolk.  Paste  tin-foil  smoothly  over  the  whole 
surface  of  the  egg.  Fasten  one  end  of  a  white  silk  thread  to  the 
egg  with  a  drop  of  melted  sealing  wax  so  that  the  egg  may  hang 
suspended  with  its  greater  diameter  horizontal.  Three  or  four 
such  insulated  conductors  will  be  found  convenient.  Sometimes 
it  is  convenient  for  each  egg  to  have  two  thread  supports.  Place 
a  loop  or  ring  at  the  free  end  of  each  thread.  When  the  loops 
are  placed  on  a  horizontal  rod  (e.  g.,  a  piece  of  glass  tubing),  the 
greater  diameters  of  the  suspended  eggs  should  lie  in  the  same 
straight  line.  An  elongated  conductor,  like  A  B  of  Fig.  88,  may 
be  made  by  hanging  two  or  three  egg  conductors  so  that  they  are 
in  contact 

223.  A  Neutral  Line. — If  an  insulated  conductor, 
bearing  a  number  of  pith  ball  (or  paper)  electroscopes, 
be  brought  near  an  electrified  body,  C,  (Fig.  88),  but  not 
near  enough  for  a  spark  to  pass  between  them,  the  pith 
balls  near  the  ends  of  the  conductor  will  diverge,  show- 
ing the  presence  of  separated  or  uncombined  electricity. 
The  pith  balls  at  the  middle  of  the  conductor  will  not 
diverge,  marking  thus  a  neutral  line. 

This  action  will  take  place  across  a  considerable  dis- 
tance even  if  a  large  sheet  of  glass  be  held  between  A 
and  C. 


160  NATURAL  PHILOSOPHY.  §  223 

If  C  has  a  positive  charge,  the  charge  at  A  will  be 
negative  and  that  at  B  will  be  positive,  as  may  be  shown 
by  charging  an  electric  pendulum  and  testing  at  A  and 
B,  as  shown  in  Fig.  88. 


FIG.  88. 

If  C  be  removed  or  "  discharged  "  by  touching  it  with 
the  hand,  all  traces  of  electrical  separation  in  A  B  will 
disappear.  The  charged  pith  ball  will  be  attracted  at 
every  point  of  A  B. 

224.  Charging  a  Body  by  Induction.  — If  the 
polarized  conductor  be  touched  with  the  band,  or  other- 
wise placed  in  electric  communication  with  the  earth, 
the  electricity  repelled  by  C  (Fig.  88),  will  escape,  and 
the  pith  balls  at  B  will  fall  together.  The  electricity  at  the 
other  end  will  be  held  by  the  mutual  attraction  between  it 
and  its  opposite  kind  at  C.  The  line  of  communication 
with  the  ground  being  broken  and  the  conductor  being 
removed  from  the  vicinity  of  C,  the  former  will  be  found 
charged  with  electricity  opposite  in  kind  to  that  of  (7. 


§  226  ELECTRIC  CHARGES.  161 

A  body  may  be  thus  charged  by  induction  with 
no  loss  to  the  inducing  body. 

225.  Polarization  Precedes  Attraction.  —  Wforn 
an  electrified   glass  rod  is  brought  near  an  insulated 
uncharged  pith  ball  (electric  pendulum),  the  pith  ball  is 
polarized,  as  shown  in  the  figure. 

As  the  —  of  the  ball  is  nearer  the 
-f-  of  the  glass  than  is  the  +  of  the 
ball,  the  attraction  is  greater  than 
the  repulsion. 

If  the  pith  ball  be  suspended,  not  pIG  g9. 

by  a  silk  thread  but  by  some  good 
conductor,  the  attraction  will  be  more  marked,  for  the 
+  of  the  ball  will  escape  to  the  earth  through  the  sup- 
port and,  thus,  the  repelling  influence  will  be  removed. 

226.  Provisional  Theory  of  Electricity.— While 
the  real  nature  of  electricity  remains  unknown,  the  fol- 
lowing theory  will  be  found  convenient  for  classifying 
results  already  attained  and  suggesting  directions  for 
further  inquiry.     But  we  must  not  let  it  influence  our 
judgment  as  to  what  is  the  true  and  full  explanation  of 
electrical  phenomena,  which  explanation  may  be  found 
hereafter : 

We  may  assume  that  a  neutral  or  unelectri- 
fied  body  contains  equal  and  equally  distributed 
quantities  of  positive  and  of  negative  electricity. 

We  may  assume  these  electricities  to  be  un- 
limited in  amount. 

We  shall  then  conceive   that   a  positively  elec- 


NATURAL   PHILOSOPHY. 


§227 


trified  body  has  an  excess  of  4-  electricity  and 
that  a  negatively  electrified  body  has  an  excess 
»f  —  electricity. 

fn  this  light,  we  shall  see  that  communicating 
4-  electricity  to  a  body  is  equivalent  to  removing 
an  equal  amount  of  —  electricity  from  it,  and 
conversely. 

227.  The  Electrophorus. — This  simple  instrument 
consists  generally  of  a  shal- 
low tinned  pan  filled  with 
resin,  oil  Aivhich  rests  a  mov- 
able metallic  cover  with  a 
glass  or  other  insulating 
handle.  The  resinous  plate 
may  be  replaced  by  a  piece 
of  vulcanized  India-rubber. 
The  metal  surface  and  the 
resinous  surface  touch  at 
only  a  few  points;  they  are 
PI  practically  separated  by  a 

Fio.  90.  thin  layer  of  air. 

(a.)  The  resinous  plate  may  be  prepared  by  melting  together 
equal  quantities  of  rosin  and  Venice  turpentine  and  then  adding  a 
like  quantity  of  shellac.  The  substances  should  be  heated  gradu- 
ally and  stirred  together  so  as  to  prevent  the  forming  of  bubbles. 
Take  care  that  the  mixture  does  not  take  fire  in  course  of  prepara- 
tion. The  Venice  turpentine  is  desirable  but  not  necessary.  For 
a  handle,  a  stout  wire  may  be  soldered  to  the  centre  of  the  disc 
and  covered  with  rubber  tubing;  or  a  piece  of  sealing  wax,  of 
convenient  size,  may  be  fastened  to  the  disc  for  the  purpose.  A 
•till  better  plan  is  to  make  the  cover  of  wood,  a  little  less  in 


ELECTRIC   CHARGES. 


163 


diameter  than  the  resinous  plate  and  with  its  edges  carefully 
rounded  off.  For  a  handle,  a  glass  rod  or  tube  may  be  tightly 
thrust  or  cemented  into  a  hole  in  the  middle  of  the  cover.  Paste 
tin  toil  all  over  the  cover  and  smooth  down  all  rough  edges  of  the 
foil  with  the  finger  nail  or  a  paper  folder.  The  wire  support  for  a 
pith  ball  or  paper  electroscope  may  be  thrust  into'the  wood  of  the 
cover,  care  being  taken  that  it  touches  the  tin -foil. 

(b.)  The  plate  is  rubbed  or  struck  with  flannel  or  catskin,  and 
thus  negatively  electrified.  The  cover  is  then  placed  upon  the 
resin  and  thus  polarized  by  induction.  Touch  tlie  cover  with  the 
fineer,  as  shown  in  Fie:.  90 ;  the  free  —  electricity  escapes  and 
the  leaves  fall.  The  cover  is  now  charged  positively.  The  charged 
cover  will  give  a  spark  to  the  knuckle  or  other  unelectrified  body 
presented  to  it.  (fig.  91.) 


228.  The  Elec- 
trophorus  charged 
by  Induction.— The 
cover  may  be  thus 
charged  and  dis- 
charged an  indefinite 
number  of  times,  in 
favorable  weather, 
without  a  second 
electrifying  of  the 
resinous  plate.  This 
could  not  happen  if 
the  electricity  of  the 
cover  were  drawn 
from  the  plate.  More- 
over, JT  v^»  ch'irsre  °^ 
the  cover  were  drawn 


lie  91 


_64  NATURAL  PHILOSOPHY.  §  22Q 

from  the  plate,  it  would  be  — ,  and  not  4* .     The  cover 
is  charged  by  induction  and  not  by  conduction. 

229.  Whence   this  Energy  ? — At  every  discharge 
of  the  electrophorus,  it  gives  a  definite  amount  of  elec- 
tricity, capable  of  doing  a  definite  amount  of  work.     As 
this  is  obtained  not  by  the  expenditure  of  any  part  of 
the  original  charge,  we  are  led  to  seek  for  the  source  of 
this  apparently  unlimited  supply  of  energy. 

"As  a  matter  of  fact,  it  is  a  little  harder  work  to  lift 
the  cover  when  it  is  charged  with  the  +  electricity  than 
if  it  were  not  charged  for,  when  charged,  there  is  the 
force  of  the  electric  attraction  to  be  overcome  as  well  as 
ihe  force  of  gravity.  Slightly  harder  work  is  done  at 
the  expense  of  the  muscular  energies  of  the  operator  and 
this  is  the  real  origin  of  the  energy  stored  up  in  the 
separate  charges." 

230.  A  Charge  Resides  on  the  Surface.— Many 
experiments  have  been   made   showing   that   when   a 
conductor   is   electrified,  the   electricity   passes   to 
the  surface  and  escapes  if  the  body  be  not  insu- 
lated. 

Experiment  92. — Place  a  carrot  horizontally  upon  an  insulating 
support.  Into  one  end  of  the  carrot  stick  a  sewing-needle.  Bring 
the  electrified  glass  rod  near  the  point  of  the  needle  without 
touching  it.  The  —  electricity  of  the  carrot  quietly  escapes  front 
the  point  to  the  rod  and  the  carrot  is  charged  with  the  +  elec, 
tricity  that  remains. 

231.  Density.  —  Experiments   show  that  when  a 
spherical  conductor  is  charged,  the  electricity  is  evenlt 
distributed  over  jtfce.  SJK£;&@,  ggpvided  no  other  electrified 


§  232  ELECTRIC  CHARGES.  165 

body  be  near.  The  electric  density  is  the  same  at  ever^ 
point. 

Experiments  on  an  elongated  cylinder,  like  the  prime 
conductor  of  the  electric  machine,  show  that  the  density 
is  greater  at  the  ends.  On  an  egg-shaped  conductor, 
like  that  shown  in  Fig.  92,  the  density  is  greatest  at  the 
smaller  end. 

In  general,  the  electric  density  is  very  great  at 
any  pointed  part  of  a  charged  conductor. 


FIG.  92. 

This  density  at  a  point  may  become  so  great  that  the 
electricity  will  escape  rapidly  and  quietly,  the  air  par- 
ticles rapidly  carrying  off  the  charge  by  convection. 
This  explains  the  action  of  points,  which  plays  so  im- 
portant a  part  in  the  action  of  electric  machines. 

232.  Electric  Machines.  —  Machines  have  been 
made  for  developing  larger  supplies  of  electricity  more 
easily  than  can  be  done  with  a  rod  of  glass  or  sealing- 
wax  or  with  the  electrophorus.  Each  of  them  consist; 
of  one  part  for  producing  the  electricity  and  anoth' 
part  for  collecting  it. 


166 


NATURAL   PHILOSOPHY. 


233.  The  Plate  Electric  Machine.— This  instru- 
ment is  represented  in  Fig.  93.  It  consists  of  an  insu- 
lator (or  electric),  a  rubber,  a  negative  and  a  positive 
or  prime  conductor.  The  electric  is  a  glass  (or  ebonite) 
plate,  A.  generally  one,  two  or  three  feet  in  diameter. 


FIG.  93. 

This  plate  has  an  axis,  B,  and  handle,  C,  and  is  sup- 
ported upon  two  upright  columns.  The  rubber,  D,  13 
made  of  two  cushions  of  silk  or  leather,  covered  with 
amalgam  (see  §  197,  a).  They  press  upon  the  sides  of  the 
plate  and  are  supported  from  the  negative  conductor, 
with  which  they  are  in  electric  connection. 

The  negative  conductor,  N,  is  supported  upon  an 
insulating  column  and,  when  only  positive  electricity  is 
desired,  is  placed  in  electrical  connection  with  the  earth 
by  means  of  a  chain  or  wire,  W.  The  prime  conductor, 
P,  is  insulated,  One  end  of  the  prime  conductor  termi- 


§  234  ELECTRIC   CHARGES.  167 

nates  in  two  arras,  F,  which  extend  one  on  either  side  ol 
the  plate.  These  arms,  being  studded  witli  points  pro- 
jecting toward  the  plate,  are  called  combs.  The  teeth  of 
the  combs  do  not  quite  touch  the  plate.  A  silk  bag,  S, 
is  often  supported  so  as  to  enclose  the  lower  part  of  the 
plate.  All  parts  of  the  instrument  except  the  teeth  of 
the  combs  are  carefully  rounded  and  polished,  sharp 
points  and  edges  being  avoided  to  prevent  the  escape  of 
electricity  as  already  explained.  This  avoiding  of  points 
and  edges  is  to  be  regarded  in  all  apparatus  for  use  with 
electricity  of  high  potential. 

(a.)  The  pupil  may  make  a  plate  machine  without  much  ex- 
pense. A  glazier  will  cut  for  him  a  disc  of  plate  glass,  possibly 
from  a  fragment  on  hand.  The  edges  of  this  disc  may  be  rounded 
on  a  wet  grindstone.  A  hole  may  be  bored  in  the  middle  with  a 
round  file  kept  moistened  with  a  solution  of  camphor  in  turpentine. 
The  conductors,  N  and  P,  may  be  made  of  wood  covered  with 
gold  foil  or  Dutch  leaf  and  supported  on  pieces  of  stout  glass  tub- 
ing. The  prime  conductor  may  well  have  two  such  supports.  The 
arms  may  consist  of  two  stout  wires  thrust  into  the  end  of  a  prime 
conductor,  their  free  ends  being  provided  with  knobs  of  lead  or  other 
metal.  The  combs  may  be  made  by  soldering  pin  points  to  one 
side  of  each  arm.  See  that  the  gold  foil  makes  actual  contact  with 
the  metal  arms.  See  that  all  metal  parts  except  the  pin  points  are 
polished  smooth.  The  columns  that  support  the  plate  may  be 
made  of  seasoned  wood.  The  part  of  the  handle  to  which  the 
hand  is  applied  may  be  made  of  glass  or  insulated  by  covering  it 
with  rubber  tubing. 

234.  Operation  of  the  Plate  Machine. —  The 
plate  is  turned  by  the  handle.  Electric  separation  is 
produced  by  the  friction  of  the  rubbers.  The  -f  elec- 
tricity of  the  rubber  and  negative  conductor  passes  tc 


168  NATURAL   PHILOSOPHY.  §  234 

the  plate;  the  —  electricity  of  the  plate  passes  to  the 
rubber  and  negative  conductor.  The  part  of  the  plate 
thus  positively  charged  passes  to  the  combs  of  the  prime 
conductor.  The  +  of  the  plate  acts  inductively  upon 
the  prime  conductor,  polarizes  it,  repels  the  +  and  at- 
tracts the  —  electricities. 

Some  of  the  —  electricity  thus  attracted  streams  from 
the  points  of  the  combs  against  the  glass,  while  some  of 
the  +  electricity  of  the  glass  escapes  to  the  prime  con- 
ductor. This  neutralizes  that  part  of  the  plate  and  leaves 
the  prime  conductor  positively  charged. 

The  rubber  and  negative  conductor  are  kept  in  equi- 
librium by  means  of  their  connection  with  the  earth. 
As  the  plate  revolves,  the  lower  part,  passing  from  N  to 
P,  is  positively  charged ;  the  upper  part,  passing  from  P 
to  N,  is  neutralized.  If  negative  electricity  be  desired, 
the  chain  or  other  ground  connection  is  changed  from 
N  to  P,  and  the  charge  taken  from  N. 

NOTE. — Other  forms  of  electric  machines  are  made.  One  of 
the  latest  of  these,  known  as  the  Toepler-Holtz,  is  very  compact 
and  efficient  and  remarkably  free  from  the  limitations  of  atmos- 
pheric conditions.  It  may  be  described  as  a  continuously  acting 
electrophoras  (§  227).  A  very  good  one  may  be  bought  of  JAMES 
W.  QUEEN  &  Co.,  of  Philadelphia,  for  $25  or  more.  One  should 
be  provided  for  the  school  in  some  way  if  possible.  Any  electrical 
machine  should  be  free  from  dust  and  perfectly  dry  when  used. 
It  should  be  warmer  than  the  atmosphere  of  the  room,  that  it  may 
not  condense  moisture  from  the  surrounding  air.  The  drier  the 
atmosphere  the  better  will  be  the  action  of  the  machine. 

235-  Construction  of  the  Leyden  Jar.  —The 
Leyden  jar  consists  of  a  glass  jar,  coated  within  and  with- 


§237 


ELECTRIC   CHARGES. 


169 


out  for  about  two-thirds  its  height  with  tin-foil,  and  a 
metallic  rod,  communicating  by  means  of  a 
small  chain  with  the  inner  coat  and  terminat- 
ing above  in.  a  knob.  The  upper  part  oi  the 
jar  and  the  cork  which  closes  the  mouth  of  the 
jar  and  supports  the  rod  are  generally  coated 
with  sealing-wax  or  shellac  varnish  to  lessen 
the  deposition  of  moisture  from  the  air. 

(a.)  Select  a  candy  or  fruit  jar  of  greenish  glass  ; 
paste  tin-foil  within  and  without,  as  above  described,  pIG> 
using  flour  paste ;  thrust  a  wire  through  a  dry  cork  ; 
bend  the  wire  so  that,  when  the  cork  is  in  its  place,  the  wire  shall 
touch  the  tin-foil  on  the  side  of  the  bottle  without  tearing  it ; 
solder  the  upper  end  of  the  wire  to  a  smooth  button  or  thrust  it 
into  a  lead  bullet ;  charge  your  Leyden  jar  with  a  few  sparks  from 
the  electrophorus  and  take  a  shock. 

236.  Charging  the  Leyden  Jar. — To  charge  the 
jar,  hold  it  in  the  hand,  as  shown  in  Fig.  95,  and  bring 
the  knob  near  or  into  contact  with  the  prime  conductor 
of  an  electrical  machine  which  is  in  action. 


FIG.  95. 

237.    Discharging    the    Leyden   Jar.  —  The  jar 

might  be  discharged  by  touching  the  knob  with  the 


NATURAL  PHILOSOPHY.  §  237 

In  this  case  the  experimenter  will  feel  a  "  shock." 
If  the  charge  be  intense,  the  shock  will  be 
painful  or  even  dangerous.  It  is  better 
to  use  a  "discharger,"  one  form  of  which 
is  represented  in  Fig.  96.  This  consists 
of  two  metal  arms  hinged  together,  carry- 
ing knobs  at  their  free  ends  and  carried  by 
insulating  handles.  The  outer  coat  should 
FlG-  96'  be  touched  first. 

(a.)  A  good  discharger  may  be  made  by  passing  a  piece  of  stout 
copper  wire,  about  a  foot  long,  through  a  piece  of  rubber  tubing 
and  providing  a  metal  knob  for  each  end  of  the  wire.  The  flexi- 
bility of  the  wire  avoids  the  necessity  for  a  hinged  joint. 

238.  Modes  of  Discharge. — An  electrified  con- 
ductor may  be  discharged  in  at  least  three  ways,  viz.,  by 
the  disruptive  discharge,  by  the  conveciive  discharge 
and  by  the  conductive  discharge. 

The  discharge  in  any  of  these  ways  is  accom- 
panied by  a  transformation  of  energy,  sound, 
light,  heat,  chemical  action  and  other  phenom- 
ena being  produced. 

Experiment  93.— Present  a  knuckle  of  the  hand  or  a  metal 
knob  to  the  prime  conductor  of  an  electric  machine  and  "  draw 
sparks"  therefrom. 

239.  The  Disruptive  Discharge. — A  discharge  of 
electricity  taking  place  suddenly  through  a  non-con- 
ductor is  called  a  disruptive  discharge,  e.  g.,  the  sparki 
drawn  from  an  electric  machine  in  action. 


§  242  ELECTRIC  CHARGES.  173 

Experiment  94. — At- 
tach a  pointed  wire  to  the 
prime  conductor  of  the 
electric  machine.  The 
flame  of  a  candle  held 
near  will  be  blown  away, 
as  shown  in  Fig.  97.  If 
the  candle  be  placed  upon 
the  prime  conductor  and 
a  pointed  conductor  be 
held  in  the  hand  near 
the  candle,  the  flame  will 
still  be  blown  away.  Fl°  97- 

240.  The  Convective  Discharge. — When  electric- 
ity of  high  potential  accumulates  with  so  great  a  density 
as  to  electrify  the  neighboring  particles  of  air  which, 
driven  by  electric  repulsion,  fly  off  carrying  part  of  the 
charge  with  them,  we  have  what  is  called  the  convective 
discharge.     Such  discharges  are  best  manifested  in  gases 
at  low  pressure,  in  tubes  exhausted  by  an  air-pump. 

241.  The  Conductive  Discharge.— The  flow  of  a 
continuous  current  of  electricity  constitutes  the  con- 
ductive discharge. 

When  electricity  flows  through  a  wire  from  the  prime 
conductor  of  an  electric  machine  to  the  rubbers  or  from 
the  positive  pole  of  a  voltaic  cell  or  battery  to  the  nega- 
tive, we  have  a  conductive  discharge.  It  will  be  con- 
sidered in  the  section  especially  devoted  to  voltaic 
electricity. 

242.  Lightning. — When  an  electrified  cloud  floats 
over  the  earth,  separated  from  it  by  a  layer  of  insulating 


172  NATURAL  PHILOSOPHY.  §242 

air,  the  inductive  influence  of  the  cloud  renders  the 
ground  beneath  oppositely  electrified.  Then  the  cloud, 
ground  and  insulating  air  correspond  respectively  to 
the  inner  and  outer  coatings  and  the  insulating  glass  of 
a  Leyden  jar. 

As  the  charge  of  a  Leyden  jar  may  be  made  so  intense 
that  the  attraction  of  the  separated  electricities  will 
result  in  their  rushing  together  and  thus  piercing  the 
jar,  so  the  charge  of  a  cloud  may  become  sufficiently 
intense  to  overcome  the  resistance  of  the  air  and  a  light- 
ning stroke  ensues.  Such  electric  sparks  are  sometimes 
more  than  a  mile  in  length  but  the  duration  is  not  more 
than  0.00001  of  a  second. 

243.  Lightning-Rods.— The  value  of  lightning-rods 
depends  upon  the  tendency  of  electricity  to  follow  the 
best  conductor,  and  upon  the  effect  of  pointed  conductors 
upon  electrical  density  (§  231). 

The  lightning-rod  should  be  made  of  a  good  conduc- 
tor; copper  is  better  than  iron.  It  should  terminate 
above  in  one  or  more  points,  tipped  with  some  substance 
that  can  be  corroded  or  fused  only  with  extreme  diffi- 
culty. Platinum  or  iridium  is  a  metal  which  satisfies 
these  conditions  very  well. 

The  rod  should  extend  above  the  highest  point  of  the 
building  in  order  to  offer  the  electricity  the  shortest 
path  to  the  ground.  It  is  important  to  have  each  pro- 
jecting part  of  the  building,  as  chimneys,  towers  and 
gables,  protected  by  a  separate  rod.  All  metal  work 
about  the  roofs  or  chimneys  should  be  connected  with 
the  rod. 


§  243  ELECTRIC  CHAROES.  173 

The  rod  should  afford  an  unbroken  connection  ;  the 
joints,  if  there  be  any,  should  be  carefully  made. 

The  rod  should  terminate  below  in  water  or  in  earth 
that  is  always  inoist.  It  is  well  to  connect  the  rod  with 
underground  water-pipes  when  possible  or  with  a  large 
metal  plate.  Personal  attention  should  be  given  to  thia 
matter  when  the  rod  is  put  up  as,  being  under  ground 
and  out  of  sight,  this  part  of  the  rod  is  not  easily  inspected 
subsequently. 

A  rod  having  a  blunted  tip,  a  broken  joint  or 
terminating  in  dry  earth  is  more  dangerous 
than  no  rod  at  all.  Lightning-rod  insulators 
are  undesirable. 

(a.)  The  greatest  value  of  a  lightning-rod  is  due  to  its  quiet 
work  in  the  prevention  of  the  lightning  stroke.  Bring  the  point 
of  a  knife-blade  near  the  conductor  of  an  electric  machine  in 
operation  and  notice  the  instant  cessation  of  sparks.  The  quiet 
passage  of  electricity  from  the  earth  neutralizes  the  charge  of  the 
conductor  and  restores  the  electric  equilibrium.  In  the  same  way, 
a  lightning-rod  tends  to  restore  the  electric  equilibrium  of  the 
cloud  and  prevent  the  dangerous  discharge.  For  this  quiet  but 
very  valuable  service,  few  persons  ever  give  the  rod  any  credit. 
Every  leaf  of  the  forest  and  every  b'.ade  of  grass  is  a  pointed  coa 
iuctor  acting  in  the  same  way.  (§231.) 


174  NATURAL  PHILOSOPHY.  §  243 

NOTE. — It  is  neither  necessary  nor  very  desirable  that  all  of  the 
following  experiments  be  performed.  Several  of  them  involve 
the  same  principle  ;  but  one  school  may  have  one  piece  of  apparatus 
and  another,  another  piece.  Additional  experiments  may  be  found 
in  The  Elements  of  Natural  Philosophy. 

Experiment  95. — Figure  98  represents  the  "  electric  bells." 
The  metal  frame  is  hung  from  the  prime  con 
ductor.  The  right-hand  bell  is  suspended  by  a 
wire ;  the  other  bell  is  suspended  by  a  silk  cord 
and  connected  with  the  ground  by  means  of  a 
chain  hanging  on  the  floor.  Work  the  machine 
slowly  ;  the  clapper  vibrates  and  rings  the  bells. 
Explain. 


_       Experiment  96.  —  Place  the  two  ends  of  a 

p  "    pane  of  window  glass  upon  two  books  so  that 

the  pane  shall  be  about  two  inches  above  the 
surface  of  a  table.  Place  several  pith  balls  or  numerous  bits  of 
tissue  paper  on  the  table  under  the  glass.  Electrify  the  glass  by 
rubbing  its  upper  surface  with  silk.  Notice  the  lively  motions  of 
the  balls  or  paper  bits. 

Experiment  97. — Electrify  a  glass  rod.  Toss  a  small  sheet  of 
gold  leaf  into  the  air.  Bring  the  rod  near  the  leaf.  The  leaf  is 
drawn  toward  the  rod  and  then  thrown  off.  Chase  the  leaf  with 
the  rod  without  letting  it  touch  the  ground.  Explain. 

Experiment  98. — If  a  pupil,  standing  upon  an  insulating  stool 
(a  board  supported  by  four  warm  tumblers  will  answer)  and  hav- 
ing one  hand  upon  the  prime  conductor  of  an  electric  machine  in 
action,  bring  a  knuckle  of  the  other  hand  near  one  end  of  the 
balanced  yard-stick  Experiment  73,  it  will  follow  the  knuckle. 
Explain. 

Experiment  99.— Place  a  few  bits  of  paper  upon  the  cover  of 
the  electro>x>orus  When  the  cover  has  been  touched  with  the 


§243 


ELECTRIC   CBAKGES. 


175 


finger  and  lifted   by  the  insulating  handle,  the  paper  will  be 
thrown  ofE     Explain. 


Experiment  100. — Electrify  a  doll's 
head  covered  with  long,  dry  hair.  The 
hairs  will  stand  out,  as  shown  in  Fig. 
99,  producing  an  exaggerated  appear- 
ance of  fright. 

Experiment  101. — Vary  Experiment 
94  by  placing  the  candle  on  the  prime 
conductor  and  holding  the  point  of  a 
needle  toward  it,  as  shown  in  Fig.  100. 
The  Same  will  be  driven  away  by  the 
convective  discharge. 


FIG 


Fio.  100. 


Experiment  102.  —  Fasten 
one  end  of  a  long  fine  wire  to 
the  knob  of  the  electroscope. 
Charge  the  disc  of  the  elec- 
trophorus  and  touch  it  to  the 
other  end  of  the  wire.  Notice 
the  action  of  the  electroscope. 
Try  the  experiment  with  a  dry 
silk  thread  instead  of  the  wire. 
Describe  the  action  of  the  elec- 
troscope. What  does  this  ex- 
periment teach  about  the  wire 
and  the  thread? 


Experiment  103.— Place  an  "electric  whirl"  (which 
consists  of  a  set  of  horizontal  wire  arms  radiating  from 
a  pivot-supported  centre,  the  pointed  ends  being  all  bent 
in  the  same  direction)  upon  the  prime  conductor.  Work 
the  machine  and  the  arms  will  revolve.  (See  Fig.  101.) 


Fio.  101 


176  NATURAL  PHILOSOPHY.  §  243 

Experiment  104.— Place  a  pupil  on  the  insulating  stool.  Let 
him  hold  an  electroscope,  with  his  finger  on  the  knob.  Let  a  second 
pupil  strike  him  on  the  back  with  a  cat-skin.  Notice  the  leaves 
of  the  electroscope  at  every  stroke. 

Experiment  105.— Get  a  smooth  board  and  a  sheet  of  paper. 
Heat  them  both  before  the  fire.  Place  the  paper  on  the  board  and 
rub  it  vigorously  with  a  piece  of  india-rubber.  Remove  the  elec- 
trified paper  from  the  board  and  hold  it  near  the  wall.  It  will  fly 
to  the  wall  and  cling  to  it  for  some  time. 

Experiment  106. — Electrify  the  paper  as  in  the  last  experi 
ment.  Remove  it  from  the  board  and  hold  it  by  the  edges.  Let 
a  pupil  place  a  pith  ball  on  the  paper.  Notice  and  explain  the 
action  of  the  ball. 

Experiment  107. — Electrify  the  paper  as  in  the  last  experiment. 
While  it  is  lying  on  the  board,  cut  it  into  narrow  strips.  Take 
hold  of  all  the  strips  at  one  end  and  lift  them  from  the  board. 
Notice  the  repulsion  of  light  bodies  similarly  charged. 

Experiment  108. — Place  a  pupil  upon  an  insulating  stool  (a 
board  supported  by  four  warm  tumblers  will  answer)  and  charge 
him  by  giving  him  twenty  or  more  sparks  from  the  disc  of  the 
electrophorus.  Let  another  pupil,  not  insulated,  bring  his  knuckle 
to  any  part  of  the  body  of  the  first  pupil.  Let  the  pupils  describe 
the  result. 

Experiment  109.— Cover  one  knob  of  the  discharger  with  gun- 
cotton  sprinkled  with  powdered  rosin.  When  the  Leyden  jar  is 
discharged  with  this  discharger,  the  cotton  and  rosin  are  ignited. 

Experiment  110. — Let  a  pupil,  standing  on  an  insulating  stool, 
become  charged  by  holding  one  hand  on  the  prime  conductor 
when  the  machine  is  in  operation.  If  he  then  bring  his  knuckle 
to  a  metal  burner  from  which  a  jet  of  gas  is  issuing,  a  spark  will 
nass  between  the  knuckle  and  the  burner,  igniting  the  gas.  An 
Argand  or  Bunsen  burner  answers  well  for  this  experiment.  The 
experiment  may  be  modified  by  using,  instead  of  the  knuckle,  an 


§  244  ELECTRIC  CHARGdS.  17? 

Jcicle.held  in  the  hand.  The  gas  burner  may  be  replaced  by  a 
pupil  (not  insulated)  holding  a  spoonful  of  ether  or  chloroform 
which  readily  gives  off  an  easily  combustible  vapor. 

244.  Relation  of  Electricity  to  Energy.  —  The 
work  necessarily  performed  in  operating  an  electric 
machine  is  not  all  expended  in  overcoming  inertia  and 
friction.  Much  of  it  is  employed  in  producing  electric 
separation.  It  matters  not  whether  this  separation  be 
the  separation  of  two  fluids  or  of  something  else. 

Wliatever  be  the  nature  of  the  realities  sepa- 
rated, mechanical  kinetic  energy  is  employed  in 
the  separation  and  converted  into  the  potential 
variety  (§  100). 

An  electrified  pith  ball  or  a  charged  Leyden  jar  is 
simply  an  electrostatical  reservoir  of  potential  energy. 
In  the  discharging  of  such  a  body,  the  passage  of  the 
current  is  accompanied  by  a  loss  of  potential  energy. 
What  becomes  of  this  energy  ?  This  leads  us  to  look 
for  effects  due  to  it,  to  work  done  by  it. 

Many  illustrations  of  work  thus  done  have  been  fur- 
nished in  the  experiments  just  described.  In  every  case 
of  electric  attraction  or  repulsion,  we  have  an  evident 
reconversion  of  this  potential  energy  into  mechanical 
kinetic  energy.  We  shall  soon  see  that  the  sound,  heat 
and  light  accompanying  electric  discharges  are  forms  of 
energy  due  to  the  conversion  of  the  potential  energy  of 
electric  separation. 

We  shall  see  other  effects,  more  or  less  powerful,  when 
we  come  to  study  voltaic  and  other  forms  of  current 
electricity 


178  NATURAL  PHILOSOPHY.  §  245 

245.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


f  KINDS  AND  NAMES. 
ELECTROSTATIC  LAWS. 

ELECTROSCOPES. 

(  CONDUCTORS. 
INSULATORS. 
L  RKSISTANCB. 


3 


»- 

POTENTIAL  AND  E.  M.  F. 


CAPACITY. 

BY  CONTACT. 

ELECTRIFICATION.  •{  BY  INDUCTION.  |  " 

Electrophoru* 


DISTRIBUTION  OF  CHARGE. 

DENSITY. 


LEYDEN  JAR. 
O 


DisRUPTive. 

DISCHARGE \  CONVECTIVE. 

CONDUCTIVB. 

(  LIGHTNING. 
THUNDER-STORMS \ 

\  LIGHTNING-ROD*, 

EXPERIMENTS. 
RELATION  TO  ENERGY 


§  245  EXERCISES.  279 


EXERCISES. 

1.  Why  do  we  regard  the  two  electric  charges  produced  simul- 
taneously by  rubbing  together  two  bodies  as  being  of  opposite 
kinds? 

2.  Quickly  pass  a  rubber  comb  through,  the  hair  and  determine 
whether  the  electricity  of  the  comb  is  positive  or  negative. 

3.  Twist  some  tissue  paper  into  a  loose  roll  about  six  inches 
long.     Stick  a  pin  through  the  middle  of  the  roll  into  a  vertical 
3upport.     Present  an  electrified  rod  to  one  end  of  the  roll  and  thus 
cause  the  paper  to  turn  about  the  pin  as  an  axis.     Give  this  piece 
of  scientific  apparatus  an  appropriate  name. 

4.  (a.)  Prepare  two  wire  stirrups,  A  and  B,  like  those  shown 
in  Fig.  79,  and  suspend  them  by  threads.     Electrify  two  glass  rods 
by  rubbing  them  with  silk  and   j-lace  them  in  the  stirrups.     Bring 
A  near  B.     Notice  the  repulsion.    (6.)  Repeat  the  experiment  with 
two  sticks  of  sealing-wax  that  have  been  electrified  by  rubbing 
with  flannel.     Notice  the  repulsion,     (c.)  Place  an  electrified  glass 
rod  in  A  and  an  electrified  stick  of  sealing-wax  in  B.     Notice  the 
attraction.  •  Give  the  law  illustrated  by  these  experiments. 

5.  Why  is  it  desirable  that  a  glass  rod  used  for  electrification 
be  warmer  than  the  atmosphere  of  the  room  where  it  is  used  ? 

6.  Electrify  one  insulated  egg-shell  conductor  (§  222,  6).    Bring 
it  near  a  second  conductor  but  not  into  contact  with  it.    Touch  the 
second  egg-shell  with  the  finger,     (a.)  Experimentally,  determine 
whether  the  second  egg-shell  is  electrified  or  not.    (6.)  If  you  find 
that  it  is,  what  word  explains  the  method  of  charging?     (e.)  If 
the  second  egg-shell  is  charged,  will  its  potential  and  the  potential 
of  the  first  be  of  the  same  or  of  opposite  signs  1 


180  NATURAL  PHILOSOPHY.  §  246 

SECTION        III. 

VOLTAIC    AND    THERMO-ELECTRICITY. 

246,  Chemical    Action.  —  All    chemical    changes 
(§  11,  a,)  are  accompanied  by  electric  separation.     The 
chemical  action  between  liquids  and  metals  gives  results 
the  most   satisfactory.      Electricity   thus  developed   is 
called  voltaic  or  galvanic  electricity. 

247.  Current  Electricity.— The  principal  classes  of 
electric  currents  are  as  follows  : 

( 1.)  Currents  produced  by  chemical  action,  i.  e., 
voltaic  currents. 

(2.)  Currents  produced  by  heat,  i.  e.,  thermo- 
electric currents. 

(3.)  Currents  produced  by  other  electric  cur- 
rents or  by  magnets,  i.  e.,  induced  cur- 
rents. 

(a.)  We  have  seen  that,  when  a  body  having  an  electrical  charge 
is  properly  connected  with  another  of  lower  potential,  there  is  a 
transfer  of  electricity  from  the  former  to  the  latter.  This  implies 
that  there  is  an  electric  current.  But  this  current  is  only  mo- 
mentary and  of  little  importance  in  comparison  with  the  currents 
that  we  are  about  to  consider. 

(J>.)  Current  electricity  may  differ  from  static  electricity  in  quau 
tity,  electromotive  force,  etc.,  but  not  in  its  nature. 


§249 


VOLTAIC  ELECTRICITY. 


181 


248.    The    Voltaic    Current. 

copper  and  one  of  zinc  are  placed 
in  dilute  sulphuric  acid  or  in  a 
battery  solution  like  the  one 
already  used,  the  two  strips  being 
connected  above  the  acid  by  a  wire 
conductor,  a  current  of  electricity 
is  produced.  The  apparatus  here 
described  is  called  a  voltaic  or 
galvanic  element  or  cell. 


When  a  strip  of 


FIG.  102. 


(a.)   For  voltaic  purposes,   the  sul- 
phuric acid  should  be  diluted  by  slowly 

pouring  the  acid  into  ten  or  twelve  times  its  bulk  of  soft  water. 
Do  not  pour  the  water  into  the  acid. 

249.  Direction  of  the  Current. — The  metal  most 
vigorously  acted  upon  by  the  liquid  constitutes  the  gen- 
erating or  positive  plate ;  the  other,  the  collecting  or 
negative  plate. 

When  the  wires  from  the  two  plates  are  in  contact,  it 
is  said  that  the  circuit  is  closed;  when  the  plates  are  not 
thus  in  electric  connection,  it  is  said  that  the  circuit  is 
broken. 

When  the  circuit  is  broken,  the  ends  of  the  wires  are 
called  poles  or  electrodes.  The  negative  pole  is  at- 
tached to  the  positive  plate  and  vice  versa.  Strips  of 
platinum  are  often  fastened  to  the  ends  of  the  wires; 
these  platinum  strips  then  constitute  the  electrodes. 

In  the  liquid,  the  current  is  from  the  +  to  the 
—  plate.  In  the  wire,  the  current  is  from  the 


182  NATURAL   PHILOSOPHY.  §  2$Q 

+  to  the  —  electrode.     In  each   case,  the   current 
passes  from  -f  to  — . 

The  direction  of  the  current  is  indicated  by  the  arrows 
in  Fig.  102. 

250.  Internal  Resistance. — We  may  imagine  that 
the  two  plates  of  a  voltaic  cell  are  connected  by  a  liquid 
prism.     The  greater  the  distance  between  the  plates,  the 
longer  this  prism  and  the  greater  its  resistance.     The 
larger  the  plates,  the  larger  the  prism  and  the  less  its 
resistance. 

Gases  are  poor  conductors.  Hence,  the  hydrogen 
bubbles  that  often  adhere  to  the  negative  plate  increase 
the  internal  resistance  of  the  cell  by  lessening  the  effec- 
tive surface  of  the  plate.  (§  272.) 

251.  The  Ampere. — The  strength  of  current  or  its 
rate  of  flow  will  depend  upon  electromotive  force  and 
resistance,  increasing  with  the  former  and  decreasing 
with  the  latter. 

T/ie  unit  of  current  is  called  an  ampere. 

(a.)  At  any  given  instant,  the  current  is  the  same  at  every  part 
of  the  circuit. 

252.  Ohm's  Law.  —  The  following  important  for- 
mula is  known  as  Ohm's  law  : 

r°Us  =  Amperes,  or  2=0. 


Ohms~  R 

(a.)  If  we  have  a  difference  of  potential  that  secures  an  E.  M.  F. 
of  18  volls  and  if  the  total  resistance  of  the  circuit  be  3  ohms,  the 
strength  of  the  current  will  be  6  amperes.  18  •+•  3  =  6. 


§  255  VOLTAIC  ELECTRICITY.  183 

253.  The  Coulomb.  —  The   unit   of  quantity  is 
called  the  coulomb.     It   is  the   quantity  of  elec- 
tricity  given    by   a   one   ampere   current   in   one 
.econa. 

(d.)  A  10  ampere  current  will  give  30  coulombs  in  3  seconds. 

254.  Amalgamating  the  Zinc.  —  Ordinary  com- 
mercial zinc  is  far  from  being  pure.    The  chemically  pure 
metal  is  expensive.      When  impure  zinc  is  used,  small 
closed  circuits  are  formed  between  the  particles  of  foreign 
matter  and  the  particles  of  zinc.    This  local  action,  which 
takes  place  even  when  the  circuit  of  the  cell  or  battery 
is  broken,  rapidly  destroys  the  zinc  plate  and  contributes 
nothing  to  the  general  current.      This  waste  is  pre- 
vented by  frequently  amalgamating  the  zinc.      This  ia 
done  by  cleaning  the  plate  in  dilute  acid  and  then  rub- 
bing it  with  mercury.     See  Elements  of  Natural  Phi- 
losophy, §  386,  a. 

255.  Polarization.— It  was  stated  in  §  250  that  the 
accumulation  of  hydrogen  bubbles  at  the  negative  plate 
increases  the  internal  resistance  of  the  cell.     But  the 
hydrogen  affects  the  current  in  another  way.     It  acts 
like  a  positive  plate  (being  almost  as  oxidizable  as  the 
zinc)  and  sets  up  an  opposing  electromotive  force  which 
tends  to  set  a  current  in  the  opposite  direction. 

A  cell  or  battery  in  this  condition  is  said  to  be 
polarized. 

Sometimes,  as  a  result  of  polarization,  the  strength  of 
the  current  falls  off  very  greatly  within  a  few  minutes 
after  closing  the  circuit 


184 


NATURAL   PHILOSOPHY. 


§256 


256.  Varieties  of  Voltaic  Cells.— All  voltaic  cellg 
belong  to  one  of  two  classes  : 

(1.)  Those  using  only  one  liquid. 
(2.)  Those  using  two  liquids. 

•  All  of  the  earlier  batteries  were  composed  of  one-fluid 
cells.   * 

257.  Smee's  Cell. — A  Smee's  cell  is  represented  by 
Fig.  103.      It  consists  of  a  platinized 

silver  plate  placed  between  two  zinc 
plates  hung  in  dilate  sulphuric  acid. 
The  hydrogen  bubbles  accumulate  at 
the  points  of  the  rough  platinum  sur- 
face and  are  more  quickly  carried  up 
to  the  surface  of  the  liquid  and  thus 
gotten  rid  of.  The  cell  has  an  electro- 
motive force  of  about  0.65  volts. 

258.  Potassium     Di-chromate 
Cell.— The  potassium  di-chromate  cell 

has  a  zinc  plate  hung  between  two  carbon  plates. 
lution  of  potassium  di-chromate  in  dilute  sulphuric  acid 
is  the  liquid  used.  The  hydrogen  is  given  an  opportunity 
for  chemical  union  as  fast  as  it  is  liberated. 

The  E.  M.  F.  of  this  cell  is  great  to  start  with  (from 
1.8  to  2.3  volts)  but  it  falls  very  quickly  when  the  external 
resistance  is  small.  It  quickly  recovers  and  may  be  used 
with  advantage  where  powerful  currents  of  short  duration 
are  wanted.  It  is  the  only  single  fluid  cell  that  is  free 
from  polarization. 


§  259  VOLTAIC  ELECTRICITY.  185 

(a.)  The  bottle  form  of  this  cell,  represented  in  Fig.  104,  is  the 
most  convenient  for  the  laboratory  or  lecture 
table.  By  means  of  the  sliding  rod,  the 
zinc  plate  may  be  raised  out  of  the  solution 
when  not  in  use.  Thus  adjusted,  the  cell 
may  remain  for  months  without  any  action, 
if  desired,  and  be  ready  at  a  moment's  notice. 

(&.)  One  of  the  best  proportions  for  the 
solution  is  as  follows  :  One  gallon  of  water, 
one  pound  of  di-chromate  of  potash,  and 
from  a  half  pint  to  a  pint  of  sulphuric  acid, 
according  to  the  energy  of  action  desired. 
A  small  quantity  of  nitric  acid  added  to  the 
solution  increases  the  constancy  of  the  bat- 
tery. 

259.    The    Leclanche    Cell.-  FlG"  104 

This  cell,  shown  in  Fig.  105,  contains  a  zinc  plate  or 
rod  and  a  porous  earthenware  cup  filled  with  carbon  and 
peroxide  of  manganese.  This  cup  replaces  the  other 
metal  plate.  The  liquid  used  is  a  solution  of  ammonium 
chloride  (sal  ammoniac)  in  water. 

This  cell  is  tolerably  constant  if  it  be  not  used  to  pro- 
duce very  strong  currents,  but  its  great  merib  is  that  it 
is  very  permanent.  It  will  keep  in  good  condition  for 
months  with  very  little  attention,  furnishing  a  current 
for  a  short  time  whenever  wanted.  It  is  much  used  for 
working  telephones,  electric  bells  (see  H  in  Fig.  105) 
and  clocks,  railway  signals,  etc. 

The  manganese  oxide  prevents  polarization  by  destroy- 
ing the  hydrogen  bubbles.  If  the  cell  be  used  continuously 
for  some  time,  the  power  of  the  cell  weakens  owing  to 
the  accumulation  of  hydrogen,  but  if  left  to  itself  it 


186 


NATURAL  PHILOSOPHY. 


§259 


gradually  recovers  as  the  hydrogen  is  oxidized.  Some- 
times the  manganese  oxide  is  applied  to  the  face  of  the 
carbon  and  the  porous  cup  dispensed  with.  This  cell 
has  an  E.  M.  F.  of  about  1.5  volts.  It  should  be  left  on 
open  circuit  when  not  in  use. 


FIG.  105- 


§261 


VOLTAIC  ELECTRICITY. 


187 


FIG.  106. 


260.  Daniell's  Cell. — This  cell  consists  of  a  copper 
plate  immersed  in  a  saturated  solution  of  copper  sulphate 
(blue  vitriol)  and  a  zinc  plate  immersed  in  dilute  sul- 
phuric acid  or  a  solution  of  zinc  sulphate  (white  vitriol). 
The  two  liquids  are  separated ; 
usually  one  liquid  is  contained  in 
a  porous  cup  placed  in  the  other 
liquid.  Large  crystals  of  copper 
sulphate  are  placed  on  a  perforated 
shelf  in  the  solution  of  copper  sul- 
phate to  keep  the  latter  saturated. 

Such  a  cell  will  furnish  a  nearly 
constant  current,  with  an  E.  M.  F. 
of  1.079  volts  and  keep  in  order  for 
a  long  time.  //  should  be  kepi  on 
closed  circuit  when  not  in  use. 

The  outer  cell  is  sometimes  made  of  copper  and  serves 
as  the  copper  plate,  as  is  shown  in  Fig.  106.  The 
hydrogen  passes  through  the 
porous  cell  and  acts  upon  the 
solution  of  copper  sulphate. 
Copper,  instead  of  hydrogen,  is 
deposited  upon  the  copper  plate. 
Polarization  is  thus  avoided. 


261.  The  Gravity  Cell.— 
This  is  a  modification  of  the 
Daniell's  cell,  no  porous  cup 
being  used.  The  copper  plate 
is  placed  at  the  bottom  of  the 
cell  and  the  zinc  plate  near  the 


FIG.  107. 


188  NATURAL  PHILOSOPHY.  §  26l 

top.  Crystals  of  copper  sulphate  are  piled  upon  the  cop- 
per plate  and  covered  with  a  saturated  solution  of  copper 
sulphate.  Water  or,  preferably,  a  weak  solution  of  zinc 
sulphate  rests  upon  the  blue  solution  below  and  covers 
the  zinc  plate.  The  two  solutions  are  of  different  specific 
gravities  and  remain  clearly  separated  if  the  cell  be  kept 
on  closed  circuit  when  not  in  use.  (Fig.  107.) 

This  cell  is  very  largely  used  in  working  telegraph 
lines.  It  is  sometimes  called  the  Callaud  cell. 

262.  Grove's  Cell. — The  outer  vessel  of  a  Grove's 
ceil  contains  dilute  sulphuric  acid.     In  this  is  placed  a 
hollow  cylinder  of  zinc.     Within  the  zinc  cylinder  is 
placed  a  porous  cup  containing  strong  nitric  acid.      The 
negative  plate  is  a  strip  of  platinum  placed  in  the  nitric 
acid.     The  hydrogen  passes  through  the  porous  cup  and 
reduces  the   nitric  acid   to    nitrogen    peroxide,   which 
escapes  as  brownish-red  fumes.     These  nitrogen  fumes 
are  disagreeable  and  injurious;   it  is  well,  therefore,  to 
place  the  battery  in  a  ventilating  chamber  or  outside  the 
experimenting  room. 

The  E.  M.  F.  of  the  Grove  cell,  under  favorable  con- 
ditions, is  nearly  two  volts,  while  its  internal  resistance 
is  small,  being  about  one-fifth  that  of  a  Daniell's  cell. 

It  is  much  used  for  working  induction  coils  (§  306), 
for  generating  the  electric  light,  etc.  It  is,  however, 
troublesome  to  fit  up  and  should  have  its  liquids  renewed 
every  day  that  it  is  used.  Fig.  109  represents  a,  Grove's 
battery  with  cells  joined  in  series. 

263.  Bunsen's  Cell.— Bunsen's  cell  (Fig.  108)  differs 


§264 


VOLTAIC  ELECTRICITY. 


189 


from  Grove's  in  the  use  of  carbon  instead  of  expensive 
platinum  for  the  negative  plate,  thus  reducing  the  cost. 
The  plates  are  made  larger 
than  for  Grove's  battery. 

Its  E.  M.  F.  is  about  the 
same  as  that  of  the  Grove 
cell  but  its  internal  resist- 
ance is  greater.  Fig.  110 
represents  a  battery  of  Bun- 
sen's  cells  joined  in  multiple 


FIG.  108. 


264.    A   Voltaic    Bat- 
tery.— A  number  of  vol- 
taic  elements    connected 
in  such  a  manner  that  the  current  has  the  same 
direction  in  all,  constitutes  a  voltaic  battery. 

The  usual  method  is  to  connect  the  positive  plate  of 
one  clement  with  the  negative  plate  of  the  next,  as 
shown  in  Fig.  109.  When  thus  connected,  they  are  said 
to  be  coupled  "tandem"  or  "in  series."  Sometimes  all 
of  the  positive  plates  are  connected  by  a  wire  and  all  of 
the  negative  plates  by  another  wire.  The  cells  are  then 
said  to  be  joined  "parallel,"  "abreast"  or  "in  multiple 
arc."  (See  Fig.  110.) 

(a.)  When  two  or  more  cella  are  joined  together,  the  points  of 
contact  should  be  as  large  as  is  convenient  and  kept  perfectly  clean 
The  connecting  wire  should  be  of  good  size  and,  for  the  sake  ol 
pliability,  a  part  of  it  may  well  be  given  a  spiral  form  by  winding 
it  upon  a  pencil  or  other  small  rod. 


100  NAtURAL  PHILOSOPHY  §  265 

265.  Batteries  of  High  Internal  Resistance.— 
Each  kind  of  galvanic  cell  has  an  internal  resistance,  aa 
explained  in  §  250.  A  battery  of  cells  joined  in  series 
is  called  a  "battery  of  high  internal  resistance."  (Fig. 
109.)  This  method  of  joining  the  cells  increases  the 
length  of  the  liquid  conductor  through  which  the  cur- 


FIG.  109. 

(a.)  In  a  battery  of  cells  joined  in  series,  the  E.  M.  F.  and  the 
internal  resistance  are  those  of  a  single  cell  multiplied  by  the 
number  of  cells. 

266.  Batteries  of  Low  Internal  Resistance.— 
A  battery  of  cells  joined  parallel  is  called  a  "battery  of 
low  internal  resistance."  (Fig.  110.)  This  method  of 
joining  the  cells  does  not  increase  the  length  of  the  liquid 
conductor  traversed  by  the  current  but  is  equivalent  to 
increasing  its  diameter  or  area  of  cross  section. 

For  a  circuit  of  great  external  resistance,  a  battery 
of  high  internal  resistance  is  needed.  For  a  circuit  of 


§  267  VOLTAIC  ELECTRICITY.  191 

small  external  resistance,  large  cells,  or  several  cells  joined 
parallel  are  preferable. 

(a.)  In  a  battery  of  cells  joined  parallel,  the  E.  M.  F.  is  that  of 
a  single  cell  but  the  internal  resistance  is  that  of  a  single  cell 
divided  by  the  number  of  cells. 


FIG.  110. 

(6.)  A  battery  of  high  internal  resistance  was  formerly  called 
an  intensity  battery,  while  a  battery  of  low  internal  resistance 
was  called  a  quantity  battery. 

267.  The  Best  Arrangement  of  Cells.— The  best 
method  of  coupling  cells  depends  on  the  work  to  be 
done  by  the  battery.  The  maximum  effect  is  at- 
tained when  the  internal  resistance  of  the  bat- 
tery is  equal  to  the  resistance  of  the  external 
circuit.  For  example,  suppose  that  in  a  given  battery 
of  eight  cells : 

( 1.)  Each  cell  has  an  E.  M.  F.  of  two  volts. 

(2.)  Each  cell  has  the  very  high  internal  resistance  of 
eight  ohms. 

(3.)  The  battery  is  to  work  through  a  wire  that  has  a 
resistance  of  sixteen  ohms. 


192  NATURAL  PHILOSOPHY.  §  267 

(«.)  First,  couple  the  cells  parallel.  The  E.  M.  F.  of  the  battery 
is  that  of  a  single  cell,  2  volts.  The  internal  resistance  is  8  ohms 
-5-8  =  1  ohm.  Adding  the  external  resistance,  we  have  a  total 
resistance  of  17  ohms.  (See  §  252.) 


This  arrangement  gives  us  a  current  of  0.1176+  amperes. 

(&.)  Next,  couple  the  cells  in  series.  The  E.  M.  F.  of  the  battery 
is  8  times  2  volts,  or  16  volts.  The  internal  resistance  is  8  times  8 
ohms  or  64  ohms.  Adding  the  external  resistance,  we  have  a  total 
resistance  of  80  ohms. 

C-E-      16      -02 

=  = 


This  arrangement  gives  us  a  current  of  0.2  amperes. 

(e.)  Finally,  join  the  ceils  in  two  rows  of  four  cells  each  in  series 
and  join  the  rows  parallel.  The  E.  M.  F.  of  the  battery  will  be  4 
times  2  volts  or  8  volts.  The  internal  resistance  will  be  4  times 
8  ohms  or  32  ohms  for  each  row,  but  only  half  that,  or  16  ohms, 
for  the  whole  battery.  Adding  the  external  resistance,  we  have 
a  total  resistance  of  32  ohms. 

C=  1  =  16^16  ^ 

This  arrangement,  in  which  the  internal  and  the  external  resist- 
ances are  equal,  gives  us  a  current  of  0.25  amperes,  the  greatest 
possible  under  the  given  conditions. 

(d.)  A  similar  application  of  Ohm's  law  shows  that  when  the 
external  resistance  is  large,  there  is  little  gain  from  joining  cells 
parallel  and  that  when  the  external  resistance  is  very  small,  there  it 
little  gain  in  joining  cells  in  series. 

Experiment  Ml.—  From  the  poles  of  a  potassium  di-chromate 
battery,  lead  two  stout  copper  wires  and  connect  their  free  ends  by 


§  268  VOLTAIC  ELECTKTCirr.  195 

two  or  three  inches  of  very  fine  iron  icire.  Coil  the  iron  wire 
around  a  lead  pencil  and  thrust  a  small  quantity  of  gun-cotton  into 
the  loop  thus  formed.  Plunge  the  zinc  plate  of  the  battery  into 
the  liquid  and  the  iron  wire  will  be  heated  enough  to  explode  the 
gun-cotton  ;  it  may  be  heated  to  redness  or  even  fusion. 

The  resistance  of  iron  wire  is  about  seven  times  as  great  as  that 
of  a  similar  copper  wire  ;  in  other  words,  its  conducting  power  is 
only  about  one-seventh  as  great.  The  decrease  in  the  size  of  tha 
wire  also  adds  to  its  resistance. 

268.  Thermal  Effects  of  the  Electric  Current. 

— Whenever  an  electric  current  flows  through  a  con- 
ductor, part  of  the  electricity  is  changed  into  heat. 

Electric  energy  is  changed  into  heat  energy. 
T7ie  amount  of  electricity  thus  changed  inh> 
heat  u-iU  depend  upon  the  amount  of  resistance- 
offered  by  the  conductor. 

In  the  last  experiment,  the  stout  copper  wires  wero 
good  conductors,  offered  but  little  resistance  and  con  • 
verted  but  little  of  the  electrical  energy  into  heat  energy. 
The  change  of  material  from  copper  to  iron  increased 
that  resistance.  This  increased  resistance  was  again  in- 
creased by  reducing  the  size  of  the  conductor.  For  this 
double  reason,  the  fine  wire  offered  so  much  resistance 
that  a  considerable  of  the  current  energy  was  trans- 
formed into  heat. 

Resistance  in  an  electric  circuit  always  pro^ 
duces  heat  at  the  expense  of  the  electric  current 

Thus,  electricity  is  often  used  in  firing  mines  in  milt 
tary  operations  and  in  blasting.  All  known  metals  have 
been  melted  in  this  way. 


194  NATURAL  PHILOSOPHY.  §  269 

269.  Luminous  Effects  of  the  Electric  Current. 
— When  an  electric  circuit  is  closed  or  broken,  there  is  a 
spark  at  the  point  of  contact,  due  to  the  heating  of  a  part 
of  the  conductor  to  incandescence.     We  have  seen  lumi- 
nous effects  produced  by  winding  the  wire  from  one  plate 
of  a  voltaic  cell  around  one  end  of  a  file  and  drawing  the 
other  electrode  along  the  side  of  the  file,  thus  rapidly 
closing  and  breaking  the  circuit. 

If  the  iron  wire  used  in  the  last  experiment  was  heated 
sufficiently,  it  also  gave  a  luminous  effect  and  illustrated 
the  fundamental  principle  of  the  incandescent  electric 
lamp. 

(a.)  The  most  important  luminous  effects  of  electricity  will  be 
considered  in  connection  with  dynamo  electric  machines  (§  311). 
It  will  be  noticed  that  all  of  these  are  secondary  thermal  effects. 

270.  Physiological  Effects  of  the  Electric  Cur- 
rent.— An  electric  current  may  produce  muscular  con- 
vulsions in  a  recently  killed  animal.     Experiments  with 
the  Leyden  jar  and  the  induction  coil  (§  30G)  show  that 
similar  effects  may  be  produced  upon  the  living  animal. 

Electricity  is  largely  used  as  an  agent  for  the  cure  of 
disease;  experiments  of  this  kind  may  do  injury  and 
would  better  be  left  to  the  educated  physician.  The 
discharge  of  a  large  battery  may  be  fatal  and  a  number 
of  persons  have  lost  their  lives  within  the  last  few  years 
by  coming,  accidentally  or  otherwise,  into  the  circuit  of 
a  dynamo-electric  machine. 

271.  Chemical  Effects  of  the  Electric  Current. 

— Many  chemical  compounds  in  solution  may  be  decom- 


§272 


T8ERMO- ELECTRICITY. 


195 


posed  by  forcing  the  current  to  traverse  the  solution. 
Substances  which  are  thus  decomposed  are  called  elec- 
troll/tea ;  the  process  is  called  electrolysis ;  the  com- 
pound is  said  to  be  electrolyzed. 

The  electrolysis  of  acidulated  water  is  easily  accom- 
plished with  a  current  from  two  Grove's  or  Bunsen's 
cells.  (See  Chemistry,  Experiment  12.)  The  water  is 
decomposed  into  oxygen  and  hydrogen. 

The  apparatus,  shown  in  Fig.  Ill,  may  be  called  a 
water-voltameter. 


^ 


FIG.  Ill 

272.  Ions. — The  products  of  electrolysis,  like  the 
oxygen  and  hydrogen,  are  called  ions;  the  one  that 
goes  to  the  +  electrode  (or  anode)  is  called  the  aniort ; 
the  one  that  goes  to  the  —  electrode  (kathode  or  cathode) 
is  culled  the  kathioii  or  cathioti. 


196  NATURAL  PHILOSOPHY.  §  272 

Experiment  112.  — From  the  +  pole  of  a  voltaic  battery  or 
dynamo-electric  machine,  suspend  a  plate  of  copper ;  from  the  — 
pole,  suspend  a  silver  coin.  Place  the  copper  and  silver  electrodes 
in  a  strong  solution  of  copper  sulphate  (blue  vitriol).  When  the 
circuit  is  closed,  the  salt  of  copper  is  electrolyzed,  the  copper  from 
the  salt  being  deposited  upon  the  silver  coin  and  the  sulphuric 
acid  going  to  the  copper  or  +  electrode.  The  silver  is  thus 
electro-plated.  (Fig.  112.) 

The  countless  applications  of  this  process  of  depositing  a  me- 
tallic coat  on  a  body  prepared  for  its  reception,  constitute  the 
important  art  of  electro-metallurgy. 


FIG.  112. 

273.  The  E.  M.  F.  of  Polarization.— The  pro- 
ducts of  electrolysis  have  a  tendency  to  reunite  by  virtue 
of  their  chemical  affinity.  (Chemistry,  §  8.)  For  ex- 
ample, the  electrolysis  of  zinc  sulphate  gives  zinc  and 
sulphuric  acid.  But  we  now  well  know  that  the  chemical 
action  of  these  two  substances  has  an  electromotive  force 
of  its  own.  This  E.  M.  F.  of  the  ions  acts  in  opposition 
to  that  of  the  electrolyzing  current.  In  some  cases,  it 
rises  higher  than  the  E.  M.  F.  of  the  original  current  and 
reverses  the  direction  of  the  current. 


§  274  THERMO-ELECTRICITY.  197 

The  oxygen  and  hydrogen,  yielded  by  the  electrolysis 
of  water  (§  271),  tend  to  reunite  and  set  up  an  opposing 
E.  M.  F.  of  about  1.45  volts.  Thus  we  see  that  it  re- 
quires a  battery  or  cell  with  an  E.  M.  F.  of  more  than 
1.45  volts  to  decompose  water. 

TJiis  electromotive  force  of  the  ions  is  called 
the  E.  M.  F.  of  Polarization. 

It  may  be  observed  by  putting  a  galvanometer  in  the 
place  of  the  battery  of  the  water-voltameter  (Fig.  111). 
The  polarization  in  a  voltaic  cell  acts  in  the  same  way. 

(a.)  There  is  no  opposing  E.  M.  F.  of  polarization  when  the 
kathion  and  the  anode  are  of  the  same  metal.  For  example,  the 
feeblest  current  will  deposit  copper  from  a  solution  of  copper  sul- 
phate, when  ike  anode  is  a  copper  plate. 

274.  Secondary  Batteries. — When  a  voltameter  of 
an  electro-plating  bath  is  supplying  a  current  of  elec- 
tricity, as  mentioned  in  the  last  paragraph,  it  constitutes 
a  secondary  battery.  As  the  ions  do  not  reunite  when 
the  circuit  is  open,  the  energy  of  the  decomposing  cur^ 
rent  may  be  stored  up  as  energy  of  chemical  affinity. 

Wlien  ft  current  is  again  wanted,  the  circuit 
may  be  closed  and  the  energy  of  chemical  affinity 
at  once  appears  as  energy  of  electric  current. 
Secondary  batteries  are,  consequently,  often  called 
storage  batteries. 

(a.)  The  Faure  battery  consists  of  two  plates  of  sheet  lead  coated 
with  red  lead  (lead  oxide).  These  plates  are  separated  by  a  layer 
of  paper  or  cloth,  rolled  up  in  a  loose  coil  like  a  roll  of  carpet  and 
immersed  in  dilute  sulphuric  acid. 


198  NATURAL   PHILOSOPHY.  §  274 

(6.)  When  a  current  from  a  dynamo-electric  machine  or  a  vol- 
taic battery  is  sent  through  such  a  cell,  chemical  action  is  pro- 
duced. Oxygen  acts  on  the  coating  of  the  anode  plate  and  converts 
it  into  a  higher  oxide  of  lead.  Hydrogen  unites  with  the  coating 
of  the  kathode  plate  and  reduces  it  to  metallic  lead.  When  these 
changes  have  gone  as  far  as  possible,  the  battery  is  said  to  be 
"  charged."  The  charged  plates  will  remain  in  this  condition  for 
days  if  the  circuit  be  left  open. 

(e.)  By  closing  the  circuit,  the  plates  will,  at  any  time,  furnish 
a  current  until  they  are  changed  to  their  original  chemical  condi- 
tion. As  the  lead  plates  and  the  acid  are  not  rapidly  destroyed, 
the  battery  may  be  charged  and  discharged  many  times. 

(d.)  Many  serious  defects  in  the  Faure  battery  have  been  ob- 
viated in  the  Brush  battery,  which  is  the  only  one  yet  used  to  any 
considerable  extent  in  this  country.  A  Brush  dynamo-electric 
machine  (§  311)  is  operated  in  the  daytime  for  charging  the  bat- 
teries. At  night,  the  same  dynamo  may  be  used  for  operating  arc 
electric  lights  (§  313),  while  the  charged  secondary  battery  is 
furnishing  the  current  for  the  incandescent  electric  lighta  The 
E.  M.  F.  of  each  Brush  cell  is  about  two  volts.  For  electric  light- 
ing, they  are  generally  prepared  in  batteries  of  twenty  or  more 
cells. 

275.  Magnetic  Effects  of  the  Electric  Current. 
— Any  conductor  is  rendered  magnetic  by  passing  a  cur- 
rent of  electricity  through  it.  We  have  already  seen 
that  a  bar  of  soft  iron  may  be  temporarily  magnetized 
by  the  influence  of  the  voltaic  current.  It  may  be 
further  shown  by  the  action  of  the  bar  and  helix. 

(a.)  The  bar  may  be  a  straight  piece  of  stout  iron  wire  ;  the  helix 
may  be  made  by  winding  cotton  covered  copper  wire  upon  a  piece 
of  glass  tubing  large  enough  to  admit  the  wire  and  not  quite  as 
long  as  the  iron, 


§  276  THERMO-ELECTEICITT.  199 

(6.)  A  good  bclix,  convenient  for  many  purposes,  may  be  mada 
upon  an  ordinary  wooden  spool.  With  a  sharp  knife,  make  the 
shank  of  the  sp«ol  as  thin  as  possible  and  then  wind  the  spool 
full  of  insulated  copper  wire  about  as  large  as  ordinary  broom  or 
stove-pipe  wire.  The  iron  bar  must  be  small  enough  to  pass  easily 
through  the  hole  in  the  spool  and  long  enough  to  project  a  little 
way  beyond  each  end. 

(c.)  Either  of  these  helices  may  be  placed  in  the  circuit  of  a  cell 
and  held  in  a  vertical  position,  when  it  will  act  as  a  "  sucking " 
magnet.  The  movable  iron  core  will  be  held  in  mid-air  "  without 
any  visible  means  of  support." 

(d.)  The  "  helix  and  ring  armature,"  is  shown  in  Fig.  113. 
The  armature  is  of  soft  iron  divided  into  two  semi- 
circles with  brass  handles.  When  the  helix  is 
placed  in  a  closed  circuit,  the  semicircles  resist  a 
considerable  force  tending  to  draw  them  apart ; 
when  the  circuit  is  broken  they  fall  asunder  of 
their  own  weight  The  iron  ring  may  be  made 
without  handles  by  any  blacksmith.  Stout  cords 
will  answer  for  handles.  The  helix  may  be  made 
by  winding  insulated  wire  upon  a  pasteboard  cylin- 
der an  inch  or  an  inch  and  a  half  long.  There 
should  be  four  or  five  layers  of  the  wire  which  may 
be  tied  together  with  strings  passing  through  the  hole  in  the  helix. 

(e.)  Such  temporary  magnets  as  these  are  called  electro-mag- 
nets. The  subject  of  electro-magnets  will  be  further  considered 
in  §§  298-300. 

276.  The  Electric  Telegraph. — The  electric  tele- 
graph consists  essentially  of  an  electro-magnet  and  a 
"  key  "  placed  in  the  circuit  of  a  battery.  The  key  is  an 
instrument  by  which  the  circuit  may  be  easily  broken  or 
closed  at  will.  The  armature,  A,  of  the  magnet,  M,  is 


2CO 


NATURAL  PHILOSOPHY. 


§276 


FIG.  114. 


supported  by  a  spring,  S,  which  lifts  it  when  the  circuit 

is  broken.  When  the 
circuit  is  closed,  the 
armature  is  drawn 
down  by  the  attrac- 
tion of  the  magnet. 
Thus  the  armature 
may  be  made  to  vi- 
brate up  and  down 
at  the  will  of  the 
person  at  the  key. 
The  armature  may 
-•ct  upon  one  arm  of  a  lever,  the  other  end  of  which, 
being  provided  with  a  style  or  pencil,  P,  may  be  pressed 
against  a  paper  ribbon,  R,  drawn  along  by  clock-work. 

Thus  the  pencil  may  be  made  to  record,  upon  the 
moving  paper,  a  series  of  dots  and  lines  at  the  pleasure 
of  the  operator  at  the  key  perhaps  hundreds  of  miles 
away.  When  the  two  stations  are  several  miles  apart, 
one  of  the  wires  is  dispensed  with,  the  circuit  being 
completed  by  connecting  each  station  with  the  earth. 

The  inventor  of  the  practical  electric  telegraph  was  an 
American,  S.  F.  B.  Morse.  The  system  of  signals  devised 
by  him  is  given  in  the  Elements  of  Natural  Philosophy, 
§445. 

To  prevent  confusion,  a  small  space  is  left  between 
successive  letters,  a  longer  one  between  words,  and  a  still 
longer  one  between  sentences.  Telegraph  operators  soon 
oecome  so  familiar  with  this  alphabet  that  they  under- 
stand a  message  from  the  mere  clicks  of  the  lever  and 


277 


HERMO-EL  ECTR1CITY. 


201 


do  not  use  any  recording  apparatus.  Such  an  operatol 
is  said  to  "read  by  sound";  his  instrument  is  called  a 
"sounder." 

The    same   principle    of    communicating    signals  by 
making  and  breaking    an    electric 
circuit  is  used  in  fire  and   burglar 
alarms,  hotel-annunciators,  etc. 

277.  The  Galvanometer.  — 
We  have  already  seen  that  the  vol- 
taic current  has  a  marked  effect  in 
turning  the  magnetic  needle  from 
its  north  and  south  position,  tend- 
ing to  place  the  needle  at  right 
angles  to  the  direction  of  the  cur- 
rent. 

The  galvanometer  is  a  very  delicate  instru- 
ment for  detecting  the  presence  of  an  electric 
current  and  determining  its  direction  and 
strength. 

The  magnetic  needle  is  very  light  and  suspended  so 
as  to  turn  easily.  The  wire  conductor  is  insulated  and 
coiled  many  times  about  the  needle ;  the  effect  is  thus 
multiplied.  A  glass  cover  protects  the  apparatus  from 
dust  and  disturbance  by  air  currents.  The  instru- 
ment is  largely  used.  One  form  is  represented  in 
Fig.  115. 

If  the  instrument  shows  the  presence  and  direction  of 
the  current  without  measuring  its  strength,  it  is  a  gal' 
vanoscope  rather  than  a  galvanometer. 


FIG.  115. 


202 


NATURAL   PHILOSOPHY. 


§277 


Experiment  113. — Connect  an  iron  and  a  German  silver  wire  to 
the  binding  posts  of  a  delicate  galvanometer.  Twist  the  free  ends 
of  the  wires  together  and  heat  the  junction  in  the  flame  of  an 
alcohol  lamp.  The  deflection  of  the  galvanometer-needle  will 
show  that  an  electric  current  is  traversing  the  circuit.  Cool 
the  junction  with  a  piece  of  ice.  The  galvanometer  will  show 
that  a  second  current  is  flowing  in  the  opposite  direction. 

278.  Thermo-Electricity. — //  a  circuit  be  made 
of  two  metals  and  one  of  the 
junctions  be  heated  or  chilled, 
a  current  of  electricity  is  pro- 
duced. 

A  thermo-electric  pair  may  be 
made  by  soldering  together  a  bar 
of  antimony,  A,  and  one  of  bis- 
muth, B,  and  joining  their  free  ends  by  a  wire.  Several 
such  pairs  may  be  joined  to 
form  a  thermo-electric  series, 
as  shown  in  Fig.  116.  Sev- 
eral such  series  may  be  joined 
to  form  a  thermo-electric  pile, 
the  bars  being  separated  by 
strips  of  varnished  paper  and 
compactly  set  in  a  metal 
frame  so  that  only  the  sold- 
ered ends  are  open  to  view. 
The  free  end  of  the  antimony 
bar,  representing  the  -f  elec- 
trode, and  the  free  end  of  the 
bismuth  bar,  representing  the 
—  electrode,  are  connected  with  binding  screws,  which 


FIG. 


§  278  THERMO-ELECTRICITY,  203 

may  be  connected  with  a  sensitive,  short  coil  galvano- 
meter (Fig.  115).  The  thermo-electric  pile,  with  the 
addition  of  conical  reflectors,  is  shown  in  Fig.  117. 

A  change  of  temperature  at  either  exposed  face  of  the 
pile  produces  a  feeble  current  of  electricity  which  is 
manifested  by  the  movement  of  the  needle  of  the  gal- 
vanometer. The  instrument  is  much  used  in  scientific 
work  for  detecting  differences  in  temperature,  being 
much  more  sensitive  than  the  mercury  thermometer. 


204 


NATURAL  PHILOSOPHY. 


§279 


279.  Recapitulation. — To  be  ampli6ed  by  the  pupil 
for  review. 


(Smee's. 
Potassium 
Di-chromat« 
Leclanche. 

Ctll.  H 

f  Daniell's. 
Two  Liquids,  -j  g^ve'!8' 
[  Bunsen's. 

VOLTAIC. 

(  Tandem. 
Joined  \  Abreast. 
\  Best  Method 

Battery.  . 

High  Internal  Resistance. 
Low  Internal  Resistance. 

Current.. 

Direction. 
Strength. 

CHEMICAL 
ACTION. 

SIGN  OF  .  • 

Plate. 
Pole. 
Electrode. 

Anode. 
KathodQ 

RESISTANCE  — 

External. 
Internal. 

QUANTITY. 

ELECTRICITY 

LOCAL  ACTION  . 
POLARIZATION  .  . 

THERMAL  
LUMINOUS. 

i   Cause 
\  Remedy. 

(  Cause. 
•I  Remedy. 
\  £.  M.F. 

..Relation  to  Resistance' 

CURRENT 

EFFECTS..  . 

PHYSIOLGICAL. 
CHEMICAL  - 

Electro-Metallurgy. 

E.  M.  F.  of  Polariza- 
tion.                            (  Fauro's. 
Secondary  Batteries..  \   Uses^8' 
/   Advantages 

MAGNETIC. 

(  Electro-Magnets. 
1  Electric  Telegraph. 
(  Galvanometer. 

THERMO-ELECTRJCITY. 

§  279  EXERCISES.  205 


EXERCISES. 

1.  Does  dilute  acid  or  a  battery  solution  act  upon  zinc  mow 
vigorously  than  it  does  upon  copper,  or  otherwise  ? 

2.  State  three  ways  in  which  the  internal  resistance  of  a  voltaic 
cell  may  be  diminished.     State  two  ways  in  which  the  strength  of 
current  of  a  voltaic  cell  or  battery  may  be  increased. 

3.  What  is  "  local  action  "  and  how  may  it  be  prevented? 

4.  If  the  re.-istance  of  18.12  yards  of  No.  30  copper  wire  be  3.03 
ohms,  what  length  of  the  same  wire  is  there  in  a  coil,  the  resist- 
ance of  which  is  22.65  ohms? 

5.  Given  a  battery  of  five  Daniell's  cells  coupled   in  series. 
Each  cell  has  an  E.  M.  F.  of  1.1  volts  and  an  internal  resistance  of 
2.2  ohms.     The  wire  of  the  circuit  has  a  resistance  of  44  ohms. 
What  is  the  strength  of  the  current?  Ans.  -fa  ampere. 

6.  It  is  found  that  one  Daniell's  cell,  however  large,  will  not 
decompose  acidulated  water.     It  is  also  found  that  two  Daniell's 
cells,   however  small,  is  sufficient    for    continuous    electrolysis. 
Explain. 

7.  If  the  E.  M.  F.  of  a  Daniell's  cell  be  1.08  volts  and  that  of 
a  Grove's  cell  1.73  volts,  the  internal  resistance  of  the  former  being 
five  times  as  great  as  that  of  the  latter  and  the  external  circuit 
being  a  stout,  short  copper  wire,  the  resistance  of  which  is  so  small 
that  it  may  be  neglected,  show  that  the  Grove's  cell   will  give 
about  eight  times  as  strong  a  current  as  the  Daniell's. 

8.  Twenty-four  similar  cells  are  arrange:!  in  four  batteries  of 
•six  cells,  each  coupled  in  series.     These  batteries  are  joined  abreast. 
(a.)  How  will  the  E.  M.  F.  of  this  battery  compare  with  that  of  a 
single  cell?     (6.)  How  will   Us  internal  resistance  compare  with, 
that  of  a  single  cell  ? 

9.  Given  a  battery  of  100  Grove  cells,  each  having  an  E.  M.  F. 
of  2  volts  and  an  internal  resistance  of  0.25  ohms.     The  wire  of 
the  circuit  has  a  resistance  ^f  1000  ohms.     Determine  the  strength 


206  NATURAL  PHILOSOPHY.  §  279 

of  the  current  (a.)  when  the  cells  are  joined  in  series.     (6.)  When 
the  cells  are  joined  abreast.  Ans.  (a.)  .195  +  amperes. 

10.  Given  the  same  battery  as  in  the  last  exercise,  the  external 
circuit  now  being  a  short  wire  of  only  0.001  ohni  resistance.    De- 
termine the  strength  of  the  current  (a.)  when  the  cells  are  joined 
abreast,    (b.)  When  the  cells  are  joined  in  series. 

Ans.  (a.)  571.4  amperes;  (6.)  7.99  amperes. 

11.  Given  a  single  cell  like  those  mentioned  in  the  last  two 
exercises.    Join  its  poles  with  a  short,  stout  copper  wire,  which 
has  a  resistance  of  0.001  ohm.     Determine  the  current  that  it  will 
give  and  see  how  it  compares  with  the  current  of  50  such  cells 
joined  in  series,  as  mentioned  in  the  last  exercise. 

12.  Short  circuit  the  cell  mentioned  in  the  last  exercise,  i.  e., 
make  the  circuit  with  a  wire  of  so  little  resistance  that  it  may  be 
dropped  out  of  the  account.     Determine  the  strength  of  current. 
Will  joining  any  number  of  cells  joined  in  series  increase  this 
effect? 


MAGNETISM.  207 

SECTION      IV. 
MAGNETISM. 

280.  Natural  Magnets. — One  of  the  most  valuable 
iron  ores  is  called  magnetite  (Fe3  04).     Occasional  speci- 
mens of  magnetite  will  attract  filings  and  other  small 
pieces  of  iron.     Such  a  specimen  is  called  a,  load- 
stone.    It  is  a  natural  magnet. 

281.  Artificial    Magnets. — Artificial  magnets  are 
either  temporary  or  permanent.     A  temporary  magnet 
is  usually  made  of  soft  iron  and  is  called  an  electro- 
.nagnet.     A  permanent  magnet  is  usually  made  of  steel. 

Artificial  magnets  have  all  the  properties  of  natural  mag- 
nets and  are  more  powerful  and 
convenient.      They  are,  there- 
fore, preferable  for  general  use. 

The  most  common  forms  are 

.  , ,       ,  FIG.  118. 

the  straight  or  bar  magnet  and 

the  horseshoe  magnet.  The  first  of  these  is  a  straight 
bar  of  iron  or  steel ;  the  second  is  shaped  like  a  letter  U, 
the  ends  being  thus  brought  near  together,  as  shown  in 
Fig.  118. 

A  piece  of  iron  placed  across  the  two  poles  of  a  horse- 
shoe magnet  is  called  an  armature.  We  have  already 
learned  how  to  make  artificial  magnets. 

282.  Retentivity. — It  is  more  difficult  to  get  the 
magnetism  into  steel  than  into  iron.     It  is  also  more 
difficult  to  get  the  magnetism  out  of  steel  than  out  of 


208  NATURAL  PHTLOSOPHY.  §  282 

iron.  This  power  of  resisting  magnetization  or  demag- 
netization is  called  coercive  force  or  retentivity. 
The  harder  the  steel,  the  greater  its  retentivity.  Soft 
wrought  iron  has  but  little  retentivity. 


Experiment  1 14. — Roll  a  bar  magnet  in  iron  filings.  Withdraw 
the  magnet;  the  filings  cling  to  the  ends  of  the  bar  but  not 
to  the  middle. 

283.  Magnetic  Poles.— Magnetic  attraction  is  not 
evenly  distributed  throughout  the  bar. 

It  is  greatest  at  or  near  the  ends.  These  points 
of  greatest  attraction  are  called  the  poles  of  thfi 
magnet. 


§  284  MAGNETISM.  209 

It  is  impossible,  by  any  known  means,  to  develop  one 
magnetic  pole  without  simultaneously  developing  anothei 
pole  of  opposite  sign.  The  middle  of  the  magnet  doet; 
not  attract  iron  and  is  called  the  equator  or  neutral 
point. 

Experiment  115.— Bring  either  end  of  a  bar  magnet  near  the 
end  of  a  piece  of  iron,  A  B ;  the  iron 
is  attracted.      Bring  the  same  end  of 
the   magnet   near  the  middle   of   the 
iron  ;  the  iron  is  attracted.     Bring  tin: 
the  same  end  of  the  magnet  near  the 
other  end  of  the  iron ;  the  iron  is  at- 
tracted.    Repeat  the  experiments  with  ~  ~~       ~~ 
the  other  end  of  the  magnet :  in  each 
case  the  iron  is  attracted. 

284.  Attraction  between  a  Magnet  and  Iron. — 
Either  pole,  of  a  magnet  mill  attract  ordinary 


Experiment  116. — Freely  suspend  threa  bar  magnets,  A,  B  and 
C,  at  some  distance  from  each  other.  This  may  be  done  by  placing- 
each  magnet  in  a  stout  paper  stirrup  supported  by  a  cord  or  upon 
a  board  or  cork  floating  on  water.  See  Fig.  120.  When  they 
have  come  to  rest,  each  will  lie  in  a  north  and  south  line. 

Magnets  for  this  experiment  may  be  made  by  magnetizing 
(§  300)  three  stout  knitting-needles.  If  there  is  any  electric  light, 
apparatus  in  your  neighborhood  in  charge  of  a  good  natured  man, 
he  will  probably  magnetize  the  needles  for  you. 

Each  needle  may  be  suspended  by  means  of  a  triangular  piece 
of  stiff  writing-paper.  Pass  the  needle  through  the  paper  near 
the  lower  corners  ;  at  the  other  corner  affix  by  wax  the  end  of  a 
horse-hair.  The  poles  may  be  indicated  by  little  bits  of  red  and  of 
white  paper,  fastened  by  means  of  wax  to  the  ends  of  the  needlea 
Mark  the  north-seeking  poles,  —  and  the  south-seeking  poles,  +. 


210  XATtTttAL  PHILOSOPHY.  §  28$ 

285.  Characteristics  of  Magnets. — Magnets  are 
chiefly  characterized  by  the  property  of  attract- 
ing  iron  and  by  a  tendency  to  assume  a  particu- 
lar direction  of  position  when  freely  suspended. 

Experiment  117. — (a.)  Take  magnet  A  of  Experiment  116  from 
its  support,  and  bring  its  +  end  near  the  end  of  B  or  0.  Notice 
the  attraction. 

(6.)  Bring  the  +  end  of  A  near  the  +  end  of  B  or  C.  Notice 
the  repulsion. 

(c.)  Bring  the  —  end  of  A  near  the  —  end  of  B  or  C.  Notice 
the  repulsion. 

(d.)  Bring  the  —  end  of  A  near  the  +  end  of  B  or  C.  Notice 
the  attraction. 

(e.)  From  experiment  (a)  we  learned  that  the  —  ends  of  B  and  C 
were  each  attracted  by  the  +  end  of  A.  Bring  the  —  end  of  B 
near  the  —  end  of  0.  Notice  that  they  now  repel. 

(/.)  From  experiment  (b)  we  learned  that  the  +  ends  of  B  and 
C  were  each  repelled  by  the  +  end  of  A.  Bring  the  +  end  of  B 
near  the  +  end  of  C.  Notice  that  they  now  repel. 

(gr.)  In  similar  manner  show  that  the  +  end  of  B  will  attract 
the  —  end  of  0;  that  the  —  end  of  B  will  attract  the  +  end  of  C. 

Record  the  results  of  your  experiments  in  tabular  form,  thus  : 


(a.)  +  attracts  — . 
(d.)  —  attracts  +. 
etc. 


(6.)   +  repels  +. 
(c.)  —  repels  — . 


etc. 


Experiment  I! 8. — Var?  <ue  last  experiment  by  pasting  a  paper 
image  of  a  man  at  the  +  »  .ue  of  each  of  the  magnetized  needles 
and  a  paper  image  of  a  wr/nan  at  the  —  end  of  each.  Notice  that 
the  men  are  unfriendly  and  will  not  approacli  each  other ;  that  the 
women  turn  from  each  ot>  *,  but  that  the  man  and  the  wornajj 
are  attracted  toward  each  o"  er. 


§  286  MAGNETISM.  211 

Experiment  119. — Magnetize  a  number  of  fine  sewing-needles 
by  drawing  the  +  end  of  a  bar  magnet  three  or  four  times  from 
the  eye  to  the  point  of  each.  Cut  several  small  corks  into  slices 
about  an  eighth  of  an  inch  thick.  Through  each  cork  disc,  ouob 


FIG.  121. 

a  needle  up  to  its  eye  and  place  them  in  a  round  dish  of  water. 
These  little  magnets  have  their  like  poles  presented  to  each  othef 
and  they  mutually  repel.  Bring  the  bar  magnet,  with  its  +  end 
downwards,  over  the  needles;  they  will  be  driven  to  the  sides. 
Similarly,  bring  the  —  end  over  them ;  they  will  be  attracted 
toward  the  centre. 

286.  Laws  of  Magnets. —  (1.)  Every  magnet 
has  two  similar  poles;  like  poles  repel  each 
other ;  unlike  poles  attract  each  other. 

(2.)  Magnetic  force,  like  other  forms  of  attrac- 
tion and  repulsion,  varies  inversely  as  the  square 
of  the  distance. 

Experiment  120. — Dip  one  of  the  magnetized  knitting-needles 
Into  iron  filings,  as  in  Experiment  114.  Notice  that  filings  cling 
to  the  ends,  near  the  paper  discs  but  that  none  cling  to  the 
middle.  Now  break  the  needle  in  the  middle  and  dip  each  piece 


212  NATURAL  PHILOSOPHY.  §  28P. 

into  iron  filings.  Notice  that  the  unmarked  ends,  which  were  a* 
the  middle  of  the  unbroken  magnet,  now  attract  iron  filings  as 
well  as  do  the  marked  ends.  Poles  have  been  developed  in 
parts  of  the  needle  that  previously  showed  no  magnetic  at- 
traction. 

287.  Effect  of  Breaking  a  Magnet.— If  a  mag- 
net be  broken,  each  piece  becomes  a  magnet  with  two 
poles  and  an  equator  of  its  own.  These  pieces  may  lie 
repeatedly  subdivided  and  each  fragment  will  be  a  per- 
fect magnet. 


It  is  probable  that  every  molecule  has  its  jtoJrs 
or  is  polarized  and  that,  could  one  be  isolated, 
it  would  be  a  perfect  magnet. 

288.  Magnetized,  Magnetic  and  Diamagnetic 
Substances. — A  magnetized  body  is  one  that  can  bo 
made  to  repel  a  pole  of  a  freely  suspended  magnet. 

Substances  that  are  attracted  by  a  magnet  are  called 
magnetic ;  e.  g.,  iron,  steel  and  nickel. 

Substances  that  are  repelled  by  a  magnet  are  called 
diamagnetio  ;  e.  g.,  bismuth,  antimony  and  arsenic. 

Of  these,  iron  is  by  far  the  most  magnetic,  while  bis- 
muth is  the  most  diamagnetic. 

Experiment  121. — Wrap  a  bar  magnet  in  a  piece  of  cloth 
With  it,  attract  and  repel  the  poles  of  a  suspended  magnet. 


§  2QO  MAGNETISM.  213 

Experiment  122.— Repeat  the  last  experiment,  holding  a  slata 
or  sheet  of  zinc  between  the  two  magnets. 

Experiment  123. — Put  one  piece  of  the  broken  magnet  into  a 
bottle ;  cork  the  bottle  tightly.  With  it,  attract  and  repel  the 
poles  of  a  suspended  magnet. 

289.  Magnetic   Screens. — Nothing  but  a  mag- 
netic body  can  cut  off  the  action  of  a  magnet. 

Experiment  124. — Place  a  piece  of  card-board  or  rough  draw 
ing  paper  over  a  good  bar  magnet.  Sprinkle  iron  filings  upon  the 
card-board  and  tap  it  lightly.  The  iron  particles  will  move  and 
arrange  themselves  in  well  defined  curved  lines.  See  Fig.  123. 

290.  Magnetic  Field. — A  magnet  seems  to  be  sur- 
rounded by  an  atmosphere  of  magnetic  influence  called 
the  magnetic  field.     The  magnetic  curves,  shown  in 
the  above  experiments,  show  the  direction  of  the  lines 


FIG.  123. 


of  magnetic  force.  If  a  small  magnetic  needle  be 
suspended  over  the  card-board,  its  length  will  tend  to  lie 
in  the  direction  of  the  lines  of  magnetic  force  as  mapped 
out  by  the  iron  filings. 


214  NATURAL  PHILOSOPHY.  §  2QI 

The  "  magnetic  curves,"  formed  in  the  last  experiment 
are  very  interesting  and  instructive.  The  filings  in  any 
one  of  th.ese  curves  are  temporary  magnets  with  adjoin- 
ing poles  opposite  and  therefore  attracting.  By  using 
two  bar  magnets  placed  side  by  side,  first,  with  like  pules 
near  each  other,  and,  secondly,  with  unlike  poles  near 
each  other,  their  combined  effect  on  the  iron  filings  may 
be  easily  observed. 

Experiment  125. — Rub  one  end  of  a  steel  pen  against  the  end 
of  a  magnet.  Dip  the  pen  into  iron  filings  and  notice  that  the 
newly  made  magnet  has  a  pole  at  each  end.  Determine  the  sign 
of  each  of  these  poles,  as  indicated  in  Experiment  116. 

291.  Magnetization  by  Contact.—.^  bar  of  iron 
or  steel  may  be  magnetized  by  rubbing  it  against 
a  magnet. 

Pure  or  soft  iron  is  easily  magnetized  but  quickly 
ioses  its  magnetism  when  the  magnetizing  influence  is 
removed.  Hardened  steel  is  magnetized  with  more  diffi- 
culty but  retains  its  magnetism  after  the  removal  of  the 
magnetizing  influence. 

Experiment  126. — Move  the  point  of  an  unmagnetized  steel 
pen  to  and  fro  very  near  one  end  of  a  magnet  but  without  touch- 
ing it  to  the  magnet.  Dip  the  pen  into  iron  filings  and  determine 


FIG.  124. 

whether  or  not  it  has  been  magnetized.     If  it  has,  determine  the 
sign  of  each  pole,  as  in  the  last  experiment  and  notice  whether 


§  2Q2  MAGNETISM.  215 

the  point  of  the  pen  is  of  the  same  polarity  as  the  end  of  the 
magnet  near  which  it  was  moved. 

Experiment  127.— Bring  a  short  bar  of  soft  iron,  /,  very  near  a 
strong  bar  magnet,  M,  end  to  end,  as  shown  in  the  figure.  Sprinkle 
iron  filings  over  the  end  of  the  iron  bar  and  they  will  cling  as  they 
would  to  a  magnet.  The  iron  bar  is  a  magnet,  while  it  remains 
in  this  position. 

292.  Magnetic  Induction. — If  the  end  of  a  bar  of 
soft  iron  be  brought  near  one  of  the  poles  of  a  strong 
magnet,  the  iron  becomes,  for  the  time  'being,  a 
magnet. '  The  poles  of  the  temporary  magnet  will  be 
opposite  to  those  of  the  permanent  magnet,  t.  <?.,  if  the 
+  or  positive  pole  of  the  magnet  be  presented  to  the 
iron  bar,  it  will  develop  a  —  or  negative  pole  in  the 
nearest  end  of  the  iron  bar  and  a  -j-  pole  at  the  further 
end.  Bring  the  iron  bar  nearer  the  magnet  and  this 
effect  will  be  increased. 

Actual  contact  is  not  necessary,  but  when  the  iron  and 
the  magnet  touch,  the  magnetizing  force  is  the  greatest. 
If  a  steel  bar  be  used  instead  of  an  iron  bar,  it  will  be 
permanently  instead  of  temporarily  magnetized. 

The  iron  or  the  steel  is  induced  to  become  a 
magnet  by  the  influence  of  the  magnet  used.  It 
is  said  to  be  magnetized  by  induction. 

Experiment  128.— Bring  a  soft  iron  ring  to  the  end  of  a  mag- 
net. It  will  be  supported.  Bring  a  second  ring  into  contact  with 
the  first  ring  and  it  will  be  supported.  In  this  way  quite  a  num- 
ber of  rings  may  be  supported,  each  ring  being  magnetized  by 
the  bar  or  ring  magnet  above  it.  Of  course,  the  attractive  force 


216 


NATURAL  PHILOSOPHY. 


§292 


is  continually  weakening 
from  the  first  to  the  last 
ring. 

Now  support  the  upper 
ring  upon  your  finger  and 
remove  the  magnet.  Each 
ring  ceases  to  be  a  magnet 
and  the  chain  is  broken 
into  its  separate  links. 

Experiment  129.— Vary 

PIG.  125.  the    last    experiment   by 

using,  instead  of  the 
rings, 

1.  Soft  iron  nails. 

2.  Steel  sewing-needles. 
t3ee  if  there  is  any  difference  in  the  results. 

Experiment  130. — Suspend  an  iron  key  from  the  positive  end 
of  a  bar  magnet.  A  second  bar  magnet  of  about  the  same  power, 
with  its  poles  opposite,  is  moved  along  the  first  magnet.  When 
the  —  end  of  the  second  magnet  comes  over  the  key,  the  key 
drops. 


FIG.  126. 


The  first  magnet  tends  to  induce  a  —  pole  at  the  upper  end  of 
the  key.  The  second  magnet  tends  to  induce  a  +  pole  at  the  same 
point.  Hence  the  effect  of  each  magnet  neutralizes  that  of  the 


§  2p4  MAGNETISM.  217 

Experiment  13!. — Magnetize  a  piece  of  watch  spring  about  sii 
Inches  long  (easily  obtainable  at  the  watch  repairer's)  by  drawing 
it  several  times  between  the  thumb  and  the  end  of  a  magnet. 
Dip  it  into  iron  filings  Lift  it  carefully  with  its  load.  Bring  the 
poles  of  the  spring  magnet  together,  bending  the  magnet  into  a 
ring.  The  magnet  drops  its  load. 

293.  Induction   Precedes   Attraction.— We  now 

see  why  a  magnet  attracts  ordinary  iron;  it  first  magnet' 
izes  it  and  then  attracts  it.     The  attrac-  •* 

tion  between  unlike  poles  is  greater  than 
the  repulsion  between  like  poles  because 
of  the  smaller  distance  between  them. 
Compare  §  225. 

Experiment  132. — Test  a  common  fire-poker 
for  magnetism  by  bringing  a  small  magnetic 
aeedle  near  its  ends  and  seeing  whether  the  poker 
«-epels  either  pole  of  the  compass  needle  or  whether 
the  two  ends  of  the  poker  attract  different  poles 
uf  the  needle. 

Experiment  133.— If  the  poker  is  not  slightly  magnetic,  place 
it  with  its  upper  end  sloping  toward  the  south  BO  as  to  make  an 
angle  of  a  little  less  than  half  a  right  angle.  In  other  words, 
place  it  in  the  position  assumed  by  the  dipping  needle.  (§  295.) 
While  the  poker  is  in  this  position,  strike  it  a  few  blows  with  a 
wooden  block  or  mallet.  Test  it  again  for  magnetism. 

294.  The  Earth  is  a  Magnet.— The  earth  acts 
like  a  huge  magnet  in  determining  the  direction  of  com- 
pass and  dipping  needles.      Its  inductive  influence,  as 
shown  in  the  last  experiment,  strengthens  the  belief 

JO 


218 


NATURAL  PHILOSOPHY. 


§295 


that  it  has  such  action.  In  short,  many  facts  seem  to 
teach  that  the  earth  is  a  great  magnet  with  mag- 
netic poles  near  its  geographical  poles. 

Experiment  134. — By  means  of  a  fine  wire  fork,  gently  lay  one 
of  the  magnetized  sewing-needles  of  Experiment  119  on  the  surface 
of  water.  It  will  float  without  any  cork  or  similar  support  and 
will  assume  a  north  and  south  position.  It  may  be  considered  the 
needle  of  a  small  compass. 

295.  Magnetic  Needles. — A  small  bar  magnet 
suspended  in  such  a  manner  as  to  allow  it  to 
assume  its  chosen  position  is  a  magnetic  needle. 

(a.)  If  it  be  free  to  move  in  a  hori- 
zontal plane,  it  is  a  horizontal  needle  ; 
6.g.,  the  mariner's  or  the  surveyor's  com- 
pass (Fig.  128).  It  will  come  to  rest 
pointing  nearly  north  and  south.  If  the 
magnet  be  free  to  move  in  a  vertical 
plane  it  constitutes  a  vertical  or  dipping 
needle  (Fig.  130). 
FIG.  138. 

(6.)    Make  a  horizontal   needle  of  a 

piece  of  watch  spring  about  six  inches  long  and  straightened  by 
drawing  it  between  thumb  and  finger.  Heat  the  needle  to  red- 
ness in  a  flame  and  bend  it  double.  Bend  the  ends  back  into 
a  line  with  each  other,  as  shown  in  Fig.  129.  Magnetize  each  end 
separately  and  oppositely.  Wind  a  waxed 
thread  around  the  short  bend  at  the  mid- 
dle to  form  a  socket  and  balance  the 
needle  upon  the  point  of  a  sewing-needle 
thrust  into  a  cork  for  support.  A  little 
filing,  clipping  with  shears  or  loading  with 
wax  may  be  necessary  to  make  it  balance. 


Fig.  129. 


MAGNETISM. 


219 


(c.)  Make  a  dipping  needle  by  thrusting  a  knitting-n<jedl« 
through  a  cork  so  that  the  cork  shall  be  at  the  middle  of  the  needle. 
Thrust  through  the  cork,  at  right  angles  to  the  knitting-needle, 
half  a  knitting  needle,  or  a  sewing-needle,  for  an  axis.  Support  the 
ends  of  the  axis  upon  the  edges  of  two  glass  goblets  or  other  con- 
venient objects  (see  Fig.  130).  Push  the  knitting-needle  through 
the  cork  so  that  it  will  balance  upon  the  axis  like  a  scale  beam. 
Magnetize  the  knitting-needle  and  notice  the  dip. 


(d.)  A  magnetized 
sewing-needle,  sus- 
pended near  its  middle 
(at  its  centre  of  gravity) 
by  a  fine  thread  or  hair 
or  an  untwisted  fibre 
will  serve  as  a  dipping 
needle.  It  should  first 
be  suspended  so  as  to 
hang  horizontal  and 
magnetized  afterward. 
A  simple  form  of  dip- 
ping needle  is  repre- 
sented in  Fig.  180. 

Experiment   135.— 

Measure  the  angle  that 
your  dipping  needle 
makes  with  the  surface 
of  quiet  water.  The 
angle  in  question  is 
indicated  by  the  dotted  arc  of  Fig.  130. 


FIG.  130. 


296.  Inclination  or  Dip. —  The  angle  that  a 
dipping  needle  makes  with  a  horizontal  line  is 
called  its  inclination  or  dip. 


220  NATURAL   PHILOSOPHY.  §  2Q7 

At  the  magnetic  poles,  the  inclination  is  90° ;  at  the 
magnetic  equator,  there  is  no  inclination.  The  inclina- 
tion at  any  given  place  is  not  greatly  different  from  the 
latitude  of  that  place. 

Experiment  136.— Set  two  stakes  so  that  a  string  joining  them 
will  point  toward  the  North  Star.  The  string  will  run  north  and 
south  or  nearly  enough  so  for  our  purpose.  Place  a  long  magnet 
suspended  as  a  needle  under  or  over  the  string.  Looking  down- 
ward at  the  magnet  and  the  string,  it  will  probably  be  found  that 
the  needle  and  the  string  do  not  point  in  the  same  direction. 

The  North  Star  may  be  easily  found  any  evening  in  the  direc- 
tion indicated  by  "  The  Pointers"  of  the  well  known  constellation, 
"  The  Great  Dipper."  "  The  Pointers  "  are  the  two  stars  marked 
by  the  Greek  letters  a  and  ft  in  the  diagram  below. 


NORTH 
STAR 


GREAT 


*  r 
* 


297.  Declination  or  Variation.  — The  magnetic 
needle,  at  most  places,  does  not  lie  in  an  exact  north  and 
south  line, 


MAGNETISM.  221 


Tlie  angle  which  the  needle  makes  irith  the 
geographical  meridian  is  its  declination  or  va- 
riation. 

Experiment  137. — Send  a  current  of  electricity  from  the  small 
cell,  mentioned  in  Experiment  84  through  its  wire.  Pour  half  a 
teaspoonf  ul  of  iron  filings  upon  a  sheet  of  paper  and  bring  the  wire 
conductor  of  the  cell  into  contact  with  the  filings.  Notice  that 
the  filings  cling  to  the  wire  as  though  it  were  a  magnet.  Break 
the  circuit  and  notice  that  the  filings  fall  from  the  wire. 

298.  Electro-Magnets. — From  the  last  experiment, 
we  see  that  while  the  wire  conductor  was  carrying  an 
electric  current  it  had  the  properties  of  a  magnet.  "We 
have  already  seen  that  under  similar  circumstances,  the 
conductor  deflects  a  magnetic  needle  as  if  it  were  itself 
a  magnet.  In  fact,  such  a  conductor  is  a  temporary 
magnet.  The  magnetic  effect  is 
much  increased  if  a  considerable 
length  of  the  conductor  be  made  of 
cotton  covered  (insulated)  wire  and 
wound  into  a  coil,  as  shown  in 
Fig.  131.  Such  a  coil  is  a  magnet 
with  a  +  pole  at  one  end  and 
a  —  pole  at  the  other.  It  has  an 
easily  perceptible  magnetic  field. 
If  a  soft  iron  rod  or  core  be  intro- 

r  IG.  lol. 

duced   into  the  coil,   it  enters  the 

magnetic    field  of  the  coil  or   helix   and    becomes  a 

magnet 


NATURAL  PHILOSOPHY. 


S209 


This  combination  of  coil  and  core  constitutes  an 
electro-magnet  and  is  more  powerfully  magnetic  than 
the  coil  alone. 

An  electro-magnet  is  a  bar  of  iron  surrounded 
by  a  coil  of  insulated  wire  carrying  a  current 
of  electricity. 

It  may  be  made  more  powerful  than  any  permanent 
magnet  but  loses  its  power  as  soon  as  the  current  ceases 
to  flow  through  its  coil.  The  fact  that  the  magnetism 
of  this  apparatus  is  under  control  adapts  it  to  many  im- 
portant uses,  such  as  electric  bells  and  telegraphic  instru- 
ments. 

299.    Forms   of  Electro-Magnets. — The  bar  ol 

§  275,  a,  and  the  ring  of  Fig.  113,  with  their  helices, 
are  electro-magnets.  The  electro-magnet  more  often 
has  the  horse-shoe  form  shown 
in  Fig.  132,  so  that  the  attrac- 
tion of  both  poles  may  act  upon 
the  same  body  at  the  same  time. 
The  middle  of  the  bent  bar  is 
bare,  the  direction  of  the  wind- 
ings on  the  ends1  being  such 
that,  were  the  bar  straightened, 
the  current  would  move  in  the 
same  direction  round  every 
part. 

More  frequently,  the  two  he- 
lices, A  and  B,  have  separate  cores  which  are  joined 
by  a  third  straight  piece  into  which  the  ends  of  the 
tores  are  screwed.  An  armature  is  often  placed  across 


FIG. 


§  300  MAGNETISM.  223 

the  two  poles  of  the  magnet,  as  shown  in  the  figure. 
Electro-magnets  have  been  made  capable  of  supporting 
several  tons. 

(a.)  When  the  circuit  is  broken  and  the  current  thus  inter- 
rupted, the  iron  is  generally  not  wholly  demagnetized.  The  small 
magnetism  remaining  is  called  residual  magnetism.  The  residual 
magnetism  seems  to  vary  with  the  hardness  and  impurity  of  the 
iron.  The  cores  of  electro-magnets  for  some  purposes  are  made  of 
the  softest  and  purest  iron  attainable. 

300.  Making  Permanent  Magnets. — A  steel  bar 
may  be  permanently  magnetized  by  drawing  it,  from 
its  centre,  in  one  direction  over  one  pole  of  a  powerful 
magnet  and  then,  from  its  centre,  in  the  opposite  di- 
rection over  the  other  pole,  and  repeating  the  process 
a  few  times.  (Fig.  133.) 


Fio.  133. 

A  bar  of  steel  placed  within  a  helix  through  which  a 
strong  current  is  passing,  will  be  permanently  magnet- 
ized. 


224  NATURAL  PHILOSOPHY.  §30! 

A  steel  bar  may  be  magnetized  by  striking  it  on  end 
with  a  wooden  mallet  while  it  is  held  in  the  direction 
assumed  by  the  dipping  needle. 

301.    Relation   of  Magnetism  to  Energy.  — A 

magnet  is  a  reservoir  of  potential  energy.  This  energy 
is  due  to  the  e^pnraiture,  at  some  time,  of  a  definite 
amount  of  energy  of  some  kind.  By  virtue  of  its  poten- 
tial energy,  it  can  u'o  a  definite  amount  of  work  and  no 
mo-re.  For  instance,  it  may  attract  a  certain  amount  of 
iron.  When  thus  fuLv  loaded,  the  magnet  has  done  its 
full  work  and  can  do  no  more.  When  the  iron  is  torn 
from  the  magnet,  more  energy  is  expended  and  the 
magnet  thus  endowed  agai.n  with  potential  energy.  A 
magnet  lias  not  an  inexhaustible  supply  of  energy,  as 
some  have  supposed. 


302 


KECA  PITULA  TION. 


302.  Recapitulation.— To  be  amplified  by  the  pupti 
for  review. 


MAGNETS. 


NATURAL. 


Permanent . 


Temporary  or 
Electro-Magnets. 


MOLECULAR. 
POLES. 

CHARACTERISTICS. 
LAWS. 


RELATION  TO 

RETENTIVITY. 
MAGNETIC  SCREENS. 

f  BY  CONTACT. 


I  Magnetic. ) 

(  Diamagnetic ....  f 


Forma. 
How  Mad*. 

Definition. 

Advantages. 

Forms. 

Residual  Magnetic 


Substances. 


MAGNETIZATION. 


TERRESTRIAL.. 


f  Magnetic  Curvn. 

Bv  INDUCTION 4  Lines  of  Force. 

\  Precedes  Attr*cti# 
POLBS. 

Compass. 
Dipping. 
DECLINATION. 
DIP. 


MAGNETIC  NEEDLES. 


L  RELATION  TO  ENERGI 


NATURAL  PHILOSOPffT.  §  302 


EXERCISES. 

t  State  the  first  law  of  magnets  and  tell  why  you  believe  it  ia 
be  true. 

2.  When  you  break  a  bar  magnet  have  you  two  parts  of  a 
magnet? 

8.  What  is  meant  by  retentivity  ?  What  other  name  has  the 
same  thing  ? 

4.  Give  good  reasons  for  believing  that  the  earth  is  a  magnet 

5.  Mention  three  magnetic  substances. 

6.  How  can  you  obtain  a  magnet  with  a  single  pole  ? 

7.  A  dozen  steel  sewing-needles  are  hung  in  a  bunch  by  threads 
passed  through  their  eyes.     How  will  they  behave  when  hung 
over  the  pole  of  a  strong  magnet  ? 

8.  Devise  an  experiment  to  show  how  to  cut  off  the  influence 
of  a  magnet  from  a  piece  of  iron  that  is  not  far  distant. 

9.  If  one  should  carry  a  dipping  needle  from  the  north  magnetv-1 
pole  of  the  earth  to  the  south  magnetic  pole,  how  would  the  needle 
change  its  position  during  the  journey  ? 

10.  How  can  I  magnetize  an  iron  bar  without  using  a  current 
of  electricity  or  a  steel  or  iron  magnet  ? 

11.  What  is  the  difference  between  magnets  and  magnetized 
matter? 

12.  Six  sewing  needles  are  magnetized  and  thrust  vertically 
through  six  little  floats  of  cork.     They  are  placed  in  a  vessel  of 
water,  with  the  +  pole  of  each  needle  imgnet  pointing  upward. 
What  will  be  the  effect  of  holding  th*  -  pole  of  a  larger  magnet 
over  them  1 


§304  INDUCED  ELECTRICITY.  22? 

SECTION    V. 

,  INDUCED    ELECTRICITY. 

1  303.  Induced  Currents. — From  our  study  of  frio 
tional  electricity  and  magnetism,  we  are  familiar  with 
the  term  induction,  by  which  we  understand  the  influ- 
ence that  an  electrified  body  exerts  upon  a  neighboring 
unelectrified  body  or  that  a  magnetized  body  exerts  upon 
a  neighboring  magnetic  but  unmagnetized  body.  In 
1831,  Faraday  discovered  an  analogous  class  of  phe- 
nomena which  we  are  now  about  to  consider. 

An  induced  current  is  a  current  produced  in 
a  conductor  by  the  influence  of  a  neighboring 
current  or  magnet. 

A  current  used  to  produce  such  an  effect  is  called  an 
inducing  current. 

304.  Inductive  Effect  of  Closing  or  Breaking 
a  Circuit.  —  In  Fig.  134,  B  represents  a  double  coil 
made  as  follows:  On  a  hollow  cylinder  of  wood  or  card- 
board are  wound  several  layers  of  stout  copper  wire,  insu- 
lated by  being  covered  with  silk  or  cotton.  The  two 
ends  of  this  wire,  which  constitutes  the  primary  coil, 
are  seen  dipping  into  the  cups,  g  g\ 

Upon  this  coil  and  carefully  insulated  from  it,  ia 
wound  a  much  greater  length  of  finer,  insulated  copper 
ivire.  The  two  ends  of  this  wire,  which  constitutes  the 
secondary  coil,  are  seen  connecting  with  a  delicate,  long 
coil  galvanometer,  0.  Remember  that  there  is  no  elec- 


228 


NATURAL   PHILOSOPHY. 


§304 


trical  connection  between  the  two  coils.  Wires  from  the 
two  plates  of  a  voltaic  cell,  P,  dip  into  mercury  in  the 
cupsgg',  thus  closing  an  inducing  circuit  through  the 
primary  coil  of  B. 

While  this  circuit  is  closed,  the  galvanometer  is  at  rest, 
showing  that  no  current  is  passing  through  the  secondary 
coil.  By  lifting  one  of  the  wires  from  one  of  the  cups, 
the  inducing  current  is  interrupted.  At  this  instant  the 


FIG.  134. 


galvanometer  needle  is  deflected  as  by  a  sudden  impulse, 
which  immediately  passes  away.  This  movement  of  the 
galvanometer  needle  shows  the  existence  of  a  momentary 
induced  current  in  the  secondary  coil.  If  the  wire  just 
removed  from  the  cup  be  replaced  and  the  inducing  cur- 
rent thus  re-established,  the  galvanometer  needle  will  be 
momentarily  turned  in  the  direction  opposite  to  that  in 
which  it  was  previously  turned. 

When  a  current  begins  to  flow  through  th& 
primary  coil,  it  induces  a  current  in  the  sec- 
ondary coil. 


§  306  INDUCED  ELECTRICITY.  229 

W7ien  it  ceases  to  flow  through  the  primary 
coil,  a  current  flowing  in  the  opposite  direction 
is  induced  in  the  secondary  coil. 

Both  induced  currents  are  merely  momentary 
in  duration. 

305.  The  Extra  Current. — When  a  circuit  is  made 
or  broken,  each  convolution  of  a  coil  placed  in  the  cir- 
cuit acts  inductively  upon  the  other  convolutions  of  the 
coil  as  if  they  were  portions  of  two  unconnected  circuits. 

This  action  is  called  the  induction  of  a  cur- 
rent upon  itself;  the  current  thus  produced  is 
called  the  extra  current. 

(a.)  When  the  circuit  is  made,  the  extra  current  is  inverse  or 
opposite  in  direction  to  the  primary  current  and  acts  against  it. 
The  extra  current  at  the  breaking  of  the  circuit  is  direct  and  addt 
its  effect  to  that  of  the  primary  current. 

(6.)  Hence,  a  spark  is  seen  on  breaking  but  not  on  making  con- 
tact. Increasing  the  number  of  coils  or  convolutions  in  the  circuit 
will  increase  the  brilliancy  of  the  spark.  If  the  coil  has  an  iron 
core  (electro-magnet)  the  effect  is  especially  marked. 

306.  Ruhmkorff's  Coil. — The  induction  coil  is 
a  contrivance  for  producing  induced  currents  in 
i    secondary    coil    by    closing    and    opening,    in 
rapid   succession,  the  circuit  of  a  current  in  the 
primary  coil. 

The  essential  parts  are  described  in  §  304.  In  the 
complete  instrument,  the  axis  of  the  coils  is  a  bundle  of 
soft  iron  wires.  The  primary  circuit  is  rapidly  brokeij 


230 


NATURAL  PHILOSOPHY. 


307 


and  closed  by  an  automatic  interrupter,  shown  at  the 
Jeft  hand  end  of  the  coil  in  Fig.  135. 


FIG.  135. 


The  primary  coil  is  placed  in  the  circuit  of  a  voltaic 
battery.  The  current  induced  in  the  secondary  coil  is 
vi  high  potential. 


307.  Currents  Induced 
by  Change  of  Distance. — 

The  primary  coil  may  be  made 
movable. 

When  the  primary  coil, 
bearing  a  current,  is 
brought  near  or  thrust 
into  the  secondary  coil,  a 
current  is  induced  in  the 
latter. 

When  the  coils  are  sepa- 
rata!,  (i  current  flowing  in 
the  opposite  direction  is  in- 
duced in-  the,  secondary  coil. 


FIG.  136. 


§  308  INDUCED   ELECTRICITY.  231 

Tfie  induced  currents  flow  while  a  change  of 
distance  is  varying  the  inductive  effect  of  the 
primary  current. 

Removing  the  primary  coil  to  an  infinite  distance 
would  be  equivalent  to  breaking  its  circuit,  as  in  §  304. 

308,  Magneto  -  Electric  Currents.  —  We  have 
already  noticed  that  there  is  an  intimate  relation  be- 


FIG.  137. 

tween  electric  and  magnetic  action.  We  have  seen  that 
an  electric  current  may  develop  magnetism  and  have, 
perhaps,  wondered  if  magnetism  may  be  made  to  develop 
an  electric  current  Faraday  found  that  electricity  may 
be  thus  produced  ;  the  results  of  this  discovery  have  al- 
ready become  of  incalculable  commercial  importance.  If, 
instead  of  the  primary  coil  bearing  the  inducing  current, 
a  bar  magnet  be  used,  as  shown  in  Fig.  137,  the  effects 
produced  will  be  like  those  stated  in  the  last  paragraph. 


232 


NATURAL   PHILOSOPHY. 


§309 


When  the  magnet  is  thrust  into  the  interior 
)f  the  coil,  an  induced  current  will  flow  while 
the  motion  of  the  magnet  continues. 

Wlien  the  magnet  is  stationary,  the  current 
ceases  to  flow  and,  the  needle  of  the  galvano- 
meter gradually  comes  to  rest. 

When  the  magnet  is  withdrawn,  an  induced 
current  flows  in  the  opposite  direction. 

Of  course,  it  makes  no  difference  whether  the  magnet 
be  moved  toward  the  coil  or  the  coil  be  moved  toward 
the  magnet.  The  more  rapid  the  motion,  the  stronger 
will  be  the  induced  currents. 

309.  The  Inductive  Action  of  a  Temporary 
Magnet. — If  within  the  coil  a  soft  iron  bar  (or  still 
better,  a  bundle  of  straight,  soft,  iron  wires)  be  placed, 


FIG.  13& 


§  310  INDUCED   ELECTRICITY.  233 

as  shown  in  Fig.  138,  the  induced  current  may  be  more 
effectively  produced  by  bringing  one  end  of  a  permanent 
magnet  near  the  end  of  the  soft  iron.  In  this  case  the 
induced  currents  are  due  to  the  varying  magnetism  of 
the  soft  iron,  this  magnetism  being  due,  in  turn,  to  the 
inductive  influence  of  the  permanent  magnet  (§  292). 

When  the  intensity  of  the  magnetism  of  a  bar 
of  iron  is  increased  or  diminished,  currents  are 
induced  in  the  neighboring  coil. 

Similar  effects  may  be  produced  by  moving  one  pole 
of  the  magnet  across  the  face  of  the  coil  from  end  to 
end. 

310.  The  Wheel  Armature.  —  Imagine  the  soft 
iron  bar  in  the  helix  of  Fig.  138,  to  be  grooved  and  sev- 
eral times  as  long  as  the  helix  through  which  it  passes. 
Imagine  the  ends  of  this  bar  to  be  brought  together  so 
as  to  form  a  complete  iron  ring  carrying  one  helix.  If 
the  number  of  helices  upon  the  ring  be  increased  to 
eight  or  more,  we  shall  have  the  wheel  armature  shown 
(partly  wound)  in  Fig.  139. 

If  the  pole  of  a  magnet  be  passed  around  the  face  of 
this  wheel,  it  will  pass  eight  coils  of  wire  and  induce  a 
current  of  electricity  as  it  approaches  each  coil  and  an 
opposite  current  as  it  leaves  each  coil,  thus  inducing 
sixteen  currents  for  each  revolution.  Of  course,  it 
makes  no  difference  whether  the  magnet  be  permanent 
or  temporary,  whether  the  pole  of  the  magnet  mores  by. 
f/ic  coil  or  the  coil  pauses  by  the  pole  of  the  magnet. 
Then,  if  the  magnet  be  fixed  and  the  wheel  turns  upon, 


234  NATURAL  PHILOSOPHY.  §  310 

its  axis  in  such  a  way  as  to  carry  its  coils  across  the  ends 
of  the  magnets,  we  shall  be  inducing  sixteen  currents  of 
electricity  for  each  revolution  of  the  wheel.  This  is 
what  happens  in  the  operation  of  a  dynamo-electric 
machine. 

When  a  closed  circuit  conductor  moves  in  a 
magnetic  field  so  as  to  cut  across  the  lines  of 
magnetic  force  (§  290),  an  induced  current  of 


FIG.  189. 

electricity  flows  through  the  conductor  in  one 
direction  while  the  conductor  is  approaching  the 
point  of  greatest  magnetic  intensity  and  in  the 
opposite  direction  while  the  conductor  is  moving 
away  from  such  point  of  maximum  intensity. 

The  varying  magnetic  intensity  of  the  iron  core  ol 
each  moving  coil  increases  this  effect,  as  explained  \» 


§  3H  INDUCED   ELECTRICITY.  235 

§  309.  Of  course,  the  number  of  coils  on  the  armature 
may  be  more  or  less  than  eight,  or  the  armature  may  be 
of  a  form  almost  wholly  different  from  that  just  de- 
scribed but,  in  every  case,  the  principle  of  its  action  is 
as  above  stated 

311.  Dynamo-Electric  Machines. — In  the  Brush 
dynamo-electric  machine,  represented  in  Fig.  140,  a 
shaft  runs  through  the  machine  from  end  to  end,  carry- 
ing a  pulley,  P,  at  one  end,  a  commutator,  c,  at  the  other 
and  a  wheel  armature,  R,  at  the  middle.  The  armature, 
R,  carries  eight  or  more  helices  of  insulated  wire,  H H. 
As  the  shaft  is  turned  by  the  belt  acting  upon  P,  R  and 
c  are  turned  with  it.  As  R  turns  around,  it  carries  the 
eight  coils,  H  H,  rapidly  across  the  poles  of  the  four 
powerful  field  magnets,  MM. 

As  each  coil  passes  each  pole,  it  necessarily 
traverses  the  magnetic  field  and  cuts  across  the 
lines  of  magnetic  force;  consequently,  currents 
are  induced  in  the  coil. 

These  currents  are  carried  on  insulated  wires  to  the 
commutator  rings,  cc,  where  they  are  united  in  such  a 
way  as  all  to  flow  in  the  same  direction,  forming  a  con- 
tinuous current.  The  electricity  is  taken  from  cc.  by 
the  four  or  more  copper  plates,  i  i,  technically  called 
"brushes,"  then  carried  down  the  flexible  copper  strips, 
s  s,  then  passed  through  all  the  insulated  wire  of  the 
electro-magnets,  M  M,  and,  finally,  to  the  +  binding 
posts. 

Thence  the  current  passes  by  a  wire  to  the  external 


NATURAL  PHILOSOPHY. 


circuit,  e.  g.,  to  an  arc  lamp  (Fig.  142)  and  from  this  to 
a  second  lamp,  and  so  on  through  all  of  the  lamps  of 
the  circuit  and  from  the  last  lamp  back  to  the  —  bind- 
ing post  of  the  dynamo-electric  machine,  thus  making 
the  circuit  complete.  Sixty  or  more  arc  lamps  in  series 
may  be  worked  by  one  of  these  machines. 


FIG.  140. 


Dynamo-electric  machines  are  being  rapidly  introduced 
for  purposes  of  electric  lighting,  electro-plating,  motive 
power,  telegraphy,  etc.  They  are  made  in  various  forms, 
but  the  principle  underlying  the  action  of  them  all  is 
the  same  as  that  stated  in  the  last  paragraph.  After 
mastering  the  action  of  one  dynamo -electric  machine, 
tlie  pupil  will  have  little  trouble  in  understanding  the 
action  of  any  other  that  he  may  have  a  chance  to  exam- 
ine, Pynamo-eJectric  machines  are  often  called  "dyna- 


§  3^2  INDUCED   ELECTRICITY.  231} 

mos."    A  small  dynamo,  with  hand  power,  suitable  fot 
school  tise,  may  be  had  for  $30  or  more. 

(a)  If  permanent  magnets  are  used  instead  of  electro-magnets, 
ihe  machine  is  called  a  magneto-electric  instead  of  a  dynamo-elec- 
tric machine. 

(6.)  If  instead  of  expending  mechanical  energy  to  turn  the  shaft 
of  the  dynamo  and  thus  produce  an  electric  current,  we  pass  a 
strong  current  of  electricity  through  the  dynamo,  the  shaft  of 
the  dynnmo  will  be  turned  in  the  opposite  direction  and  may 
be  made  to  drive  ordinary  machinery  as  an  electric  motor.  In  the 
former  case,  we  convert  mechanical 

THE  S-WAN  ELECTRIC  LAMP.  energy  into  electric  energy ;  in  the 
latter  case,  we  convert  electric  en- 
ergy into  mechanical  energy. 

312.  Incandescent  Elec- 
tric Lamps.  —  When  a  con- 
ductor is  heated  to  incandes- 
cence by  the  passage  of  a  cur- 
rent, we  have  an  illustration  of 
the  fundamental  principle  of 
incandescent  electric  lighting. 
To  prevent  the  fusion  of  the 
conductor,  a  carbon  filament, 
about  the  size  of  a  horse-hair, 
is  used — earbon  never  having 
been  melted. 

To  prevent  the  combustion 
of  the  carbon  filament,  it  is  en- 
closed in  a  glass  globe  contain- 
ing either  a  high    vacuum  or 
FIG.  141.  only  some  inert  gas,  incapable 


NATURAL  PHILOSOPHY. 


§313 


of  acting  chemically  upon  the  carbon  at  even  the  high 
temperature  to  which  it  is  to  be  subjected. 

The  filament  is  carbonized  in  different  ways  and  given 
different  shapes  by  different  inventors.  Fig.  141  repre- 
sents the  Swan  incandescent  lamp  and  is  half  the 
actual  size. 


BRUSH   ELECTRIC   LAMP. 


313.  The  Vol- 
taic Arc.  — The 
most  brilliant  lumi- 
nous effect  of  current 
electricity  is  the  arc 
lamp.  (Fig.  142.) 
This  consists  essen- 
tially of  two  pointed 
bars  of  hard  carbon, 
generally  copper 
coated  (Experiment 
112),  placed  end  to 
end  in  the  circuit  of 
a  very  powerful  cur- 
rent. If  the  ends  of 
the  carbons  be  sep.i- 
rated  a  short  distance 
while  the  current  is 
passing,  the  carbon 
points  become  incan- 
descent and  the  cur- 
rent will  not  be  inter- 
rupted. 


Fict.  143. 


§  314  tNbUCED   ELECTRICITY.  239 

Wlien  the  carbons  are  thus  separated,  their 
tips  glow  ivith  a  brilliancy  which  exceeds  that 
of  any  other  light  under  human  control,  while 
the  temperature  of  the  intervening  arc  is  un- 
equaled  by  any  other  source  of  artificial  heat. 

The  mechanism  shown  in  the  upper  part  of  Fig.  142, 
is  for  the  purpose  of  automatically  separating  the  car- 
bons and  "  feeding "  them  together  as  they  are  burned 
away  at  their  tips  and  for  the  purpose  of  cutting  the  lamp 
out  of  the  circuit  in  case  of  any  irregularity  or  accident. 

Such  lamps  of  from  one  to  two  thousand  candle  power 
and  requiring  an  expenditure,  at  the  dynamo,  of  about 
one  horse-power  per  lamp,  are  now  quite  common. 
Lamps  of  a  hundred  thousand  candle  power  have  been 
made.  The  current  may  be  furnished  by  a  battery  of 
forty  or  more  Grove's  cells  but,  for  economical  reasons, 
it  is  almost  universally  supplied  by  a  dynamo-electric 
machine. 

314.  The  Telephonic  Current.— An  electric  cur- 
rent may  be  induced  in  a  coil  of  insulated  wire  surround 
ing  a  bar  magnet  by  the 
approach  and  withdrawal 
of  a  disc  of  soft  iron.  The 
disc  a  (Fig.  143),  is  mag- 
netized by  the  inductive 
influence  of  the  magnet 
m  (§  292).  The  disc,  thus 

magnetized,  reacts  upon  the  magnet,  m,  and  changes  the 
distribution  of  magnetism  therein.     By  varying  the  dis- 


S40  JfAtrtTRAL  pmLOsopffr.  §  315 

fcance  between  a  and  m,  the  successive  changes  in  the 
distribution  of  the  magnetism  of  m  induce  to-aud-fro 
currents  in  the  surrounding  coil  (§  309).  When  a  ap- 
proaches m,  a  current  flows  in  one  direction  ;  when  it 
recedes,  the  current  flows  in  the  opposite  direction. 

315.  The  Telephonic  Circuit.  —  If  the  wire  sur- 
rounding the  magnet  mentioned  in  the  last  paragraph 
be  continued  to  a  distance  and  then  wound  around  a 
second  bar  magnet,  as  shown  in  Fig.  144,  the  currents 


LIHf 


FIG.  144. 

induced  at  M  would  affect  the  magnetism  of  the  bar  at 
M'  (§  298)  or  the  intensity  of  its  attraction  for  the  neigh- 
boring disc  a'. 

A  vibratory  motion  in  the  disc  a  would  induce  electric 
currents  at  M\  these  currents,  when  transmitted  to  M' , 
perhaps  several  miles  distant,  would  affect  the  magnetism 
of  the  bar  there. 

When  the  current  generated  at  M  flows  in  such  a 
direction  as  to  reinforce  the  magnet  at  M',  the  latter 
attracts  a'  more  strongly  than  it  did  before.  When  the 
current  flows  in  the  opposite  direction,  it  weakens  the 
magnetism  of  M ',  which  then  attracts  a'  less.  The  disc, 


RECA  PITULA  TWtf. 


241 


therefore,  flies  back.     Thus,  the  vibrations  of  a'  are  like 
those  of  a. 

(a.)  We  have  here  the  principle  of  the  telephone,  so  far  as  elec- 
tric action  is  involved.  Further  consideration  of  this  instrument 
must  be  deferred  until  we  have  learned  more  concerning  sound. 
(See  §  335.) 

316.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


CLOSING..    I  t  Primary...  \ 

}•  PRIMARY  CIRCUIT,  i  Secondary  .   L  Coils. 
BREAKING  J  [  Ruhmkorff  J 


< 

HO 


2t 

if 


CHANGE  OF  DISTANCE  OF   PRIMARY  CIRCUIT. 
(   Telephones. 


MAGNETS. 


PERMANENT. 


TEMPORARY. 


Magneto- Electric  Machinet. 


Wheel  Armature. 

Dynamo  -  Electric 
Machines  which 
art  largely  used 
for 

Electro-  Plating. 
Incandescent 
Electric  Lignt 
ing. 

Arc  Electric 
Lighting. 
Charging  Stor 

age  Batten** 

NATURAL  PHILOSOPHY.  §  316 


EXERCISES. 

1.  A  manufacturer  has  surplus  power  at  his  mill.     How  can  he 
utilize  this  power  to  illuminate  his  residence,  two  miles  distant? 

2.  The  ends  of  a  coil  of  fine  insulated  wire  are  connected  with 
the  terminals  of  a  long  coil  galvanometer.     A  steel  bar  magnet  is 
slowly  pushed  into  the  hollow  of  the  coil  and  then  suddenly  jerked 
out.     What  actions  will  be  observed  in  the  needle  of  the  galvano- 
meter ? 

3.  Experience  showed  that  the  actual  cost  of  71  arc  electric 
lights  of  2000  nominal  candle  power  each,  was  as  follows : 

Consumption  of  carbons  per  hour $0.89 

Power  used  for  dynamo-electric  machine. .     .65 

Interest  on  cost  of  machines 30 

Attendance,  oil,  wear  and  tear,  etc. 86 

Total  cost  per  hour $2.20 

These  electric  lamps  displaced  578  gas  burners.  Ignoring  all 
considerations  except  that  of  dollars  and  cents,  reckoning  the  con- 
sumption of  gas  at  six  cubic  feet  per  hour  for  each  burner  and  the 
cost  of  gas  at  $2  per  1000  cubic  feet,  (a.)  which  light  is  the 
cheaper?  (6.)  What  is  the  difference  in  cost  per  hour?  (c.)  What 
is  the  difference  in  cost  per  year,  the  lights  being  burned  3000 
bourn  per  year  ? 


§  316  REVJEW   QUESTIONS.  243 

REVIEW     QUESTIONS. 

1.  (a..)  Describe  the  barometer,     (It.)  Describe  the  lifting-pump 

2.  What  class  of  lever  is  represented   by  a  common  wheel 
barrow  ? 

3.  What  simple  machine  is  represented  by  a 
carpenter's  chisel  ? 

4.  What  three  elements  of  work  measure  are 
involved  in  the  term  "  horse-power  "  ? 

5.  From  a  bottle,  cork  and  glass  tubing,  con- 
struct the  apparatus  shown  in  Fig.  145.     Make  all 
joints  air-tight.    Place  a  in  water  and  suck  at  6  until 
a  jet  is  formed  at  j.      Explain  the  action  of  the 
apparatus. 

6.  (a.)  What  art,  the  essentials  of  a  good  light- 
ning rod  ?    (6.)  What  is  an  anion  ? 

7.  A  circular  copper  dish  is  joined  to  the  zinc 
plate  of  a  small   battery.      Acidulated   water  is 
poured  into  the  dish.      A  wire  from  the  carbon 
plate  of  the  battery  dips  into  the  middle  of  the 
liquid.      A  few  scraps  of  cork  are  thrown  in  to 
render  visible  any  motion  of  the  liquid.     The  — 

pole  of  a  strong  bar  magnet  is  held  above  the  dish.       -pio    145 
What  is  the  effect? 

8.  What  phenomenon  resulted  from  the  greatest  difference  of 
electric  potential  that  you  ever  knew  anything  about  ? 

9.  What  property  of  matter  is  illustrated  by  the  fact  that  when 
a  stone  is  thrown  into  water,  it  will  displace  its  own  bulk  of  water  ? 

10.  What  is  the  difference  between  a  liquid  and  a  gas  ?    Whicb 
Is  a  fluid  ? 

11.  Describe  the  three  classes  of  levers. 

12.  The  E.  M.  F.  of  a  dynamo-electric  machine  furnishing  cur- 
rent for  16  arc  electric  lamps,  is  839  volts.     The  lamps  are  placed 
in  series,  each  one  having  a  resistance  of  4.56  ohms.     The  internal 
resistance  of  the  dynamo  is  10.54  ohms.     The  line  wire  has  a  re 
sistance  of  0.4  ohms.    What  is  the  strength  of  the  current  ? 


244 


NAfTJRAL  PHILOSOPHY. 


13.  When  I  rub  together  two  bodies,  which  is  developed  sooner, 
•f  or  —  electricity  ? 

14.  How  does  the  shape  of  a  conductor  affect  electric  density  ? 

15.  (a.)  What  is  Ohm's  law ?    (6.)  What  is  an  ohm? 

16.  A  ball  has  been  freely  falling  for  five  seconds,    (a.)  What 
to  its  velocity?    (6.)  How  far  has  it  fallen? 

17.  How  can  you    fire  gunpowder 
with  a  voltaic  current  ? 

18.  Find  the  current  (in   amperes) 
given   by  six    Grove   cells,  joined   as 
shown  in  Fig.  146,  assuming  each  cell 
to  have  an  E.  M.  F.  of  2  volts  and 
an  internal  resistance  of  0.30  ohms,  the 
external  resistance  being  10  ohms. 

19.  A  strip  of  paper  that  has  been 
rubbed  with  india-rubber  is   brought 
near  a  glass  rod  that  has  been  rubbed 
with  silk.     What  happens  ? 

20.  Was  the  paper  strip  mentioned 
in  the  last  question  charged  with  +  or 
—  electricity? 

21.  If  you  rub  with  flannel  a  stick  of  sealing-wax  held  in  the 
hand,  it  becomes  electrified.    If  similarly  you  rub  a  rod  of  brass  it 
4oes  not  become  electrified.    Explain  the  difference. 

22.  A  falling  body  was  known  to  move  337.68  ft.  in  a  single 
second.      How  long  had  it  been  falling  at  the  end  of  the  obser- 
vation ? 

23.  A  piston,  the  area  of  which  is  6  sq.  in.,  is  inserted  in  the 
side  of  a  vessel  of  water  at  an  average  distance  of  3  ft.  6  in.  be- 
low the  surface.      Find  the  force  that  must  be  exerted  on  the 
piston  to  keep  it  from  being  thrust  out  by  the  water. 


FIG.  146. 


CHAPTER 

SOUND. 


SECTION     I. 

NATURE,    REFRACTION    AND    REFLECTION  OF 
SOUND. 

317.  Definition  .of  Sound. — Sound  is  the  mode 
of  motion  that  is  capable  of  affecting  the  audi- 
tory nerve. 

(a.)  The  word  sound  is  used  in  two  different  senses.  It  is  often 
used  to  designate  a  sensation  caused  by  waves  of  air  beating  upon 
the  organ  of  hearing;  it  is  also  used  to  designate  these  aerial 
waves  themselves.  If  every  living  creature  were  deaf,  there  could 
be  no  sound  in  the  former  sense,  while  in  the  latter  sense  the 
sound  would  exist  but  would  be  unheard.  The  definition  above 
considers  sound  in  the  latter  or  physical  sense  only. 

318.  Undulations.— In   beginning    the   study   of 
acoustics,  it  is  very  important  to  acquire  a  clear  idea  of 
the  nature  of  undulatory  motion.     When  a  person  sees 
waves  approaching  the  shore  of  a  lake  or  ocean,  there 
arises  the  idea  of  an  onward  movement  of  great  masses 
of  water.     But,  if  the  observer  give  his  attention  to  a 
piece  of  wood  floating  upon  the  water,  he  will  notice 
that  it  merely  rises  arid  falls  without  approaching  thq 


246  NATURAL  PHILOSOPHY.  §  318 

shore-     He  may  thus  be  enabled  to  correct  his  erroneous 
idea  of  the  onward  motion  of  the  water. 

Again,  he  may  stand  beside  a  field  of  ripening  grain 
and,  as  the  breezes  blow,  he  will  see  a  series  of  waves 
pass  before  him.  But,  if  he  observe  carefully  and  re- 
flect, he  will  see  clearly  that  there  is  no  movement  of 
matter  from  one  side  of  the  field  to  the  other;  the 
grain-ladened  stalks  merely  bow  and  raise  their  heads. 
Most  persons  are  familiar  with  similar  wave  movements 
in  ropes,  chains  and  carpets. 

Each  particle  of  matter  has  a  motion,  but  that 
motion  is  vibratory,  not  progressive.  The  onward, 
movement  is  that  of  the  wave  but  not  of  the  part- 
icles which  compose  the  wave. 

It  is  a  familiar  fact  that  a  wave  may  transmit  energy. 

(a.)  The  motion  of  the  wave  must  be  clearly  distinguished  from 
the  motion  of  particles  which  constitute  the  wave.  The  wave 
may  travel  to  a  great  distance  :  the  journey  of  the  individual  par- 
ticle is  very  limited. 

Experiment  138. — Suspend  a  pith  ball  by  a  thread  so  that  it 
shall  hang  lightly  against  one  prong  of  a  tuning  fork.  When  the 
fork  is  sounded,  the  pith  ball  will  be  thrown  off  by  the  vibra' 
tions  of  the  prongs. 

Experiment  139. — Place  the  two  ends  of  a  common  friction 
match  on  convenient  supports,  as  on  two  fingers  of  the  left  hand  ot 
the  upper  edges  of  a  partly  opened  book  standing  on  end  ou  a 


§  319  CAUSE  OF  SOUND.  247 

Strike  a  olow  with  a  common  tuning  fork  to  set  its  prongs  ill 
vibration,  bring:  the  fork  near  the  ear  and  notice  the  sound; 
quickly  bring  the  broad  face  of  one  prong  beneath  the  middle  of 
the  match ;  the  match  will  be  thrown  upward  by  the  sudden 
blow. 

Experiment  140. — In  similar  manner,  sound  a  fork  and  with 
the  ends  of  the  prongs,  quickly  touch  the  surface  of  a  glassful  of 
water.  The  vibrating  prongs  of  the  sounding  fork  will  throw 
•:wo  showers  of  spray  from  the  water. 

319.  Cause  of  Sound. — All  sound  may  be  traced 
to  the  vibrations  of  some  material  body. 

The  particles  of  a  sounding  body  strike  the  adjacent 
particles  of  air,  these  pass  the  motion  thus  received  to 
the  air  particles  next  beyond,  and  these  to  those  still 
oeyond. 

Experiment  141.— Provide  a  tube  four  or  five  metres  (or  yards) 
Jong,  and  about  ten  centimeters  (four  inches)  in  diameter.  A  few 
lengths  of  common  spout  from  the  tinner's  will  answer.  Furnuh 
it  with  a  funnel  shaped  piece,  having  an  opening  about  2^  ci.i. 
(one  inch)  in  diameter.  Place  the  tube  on  a  table  with  a  candle 


flame  opposite  the  opening  at  B.  With  a  book,  strike  a  sha>p 
blow  upon  the  table  opposite  the  opening  at  A.  The  flame  will 
be  blown  out.  Something  went  from  A  to  B.  Did  it  go  through 
the  tube? 

Experiment  142,— i«et  us  ask  this  question  of  Nature,  speakir  g 


248  NATURAL  PHILOSOPHY.  §  320 

<rith  her,  as  we  must,  in  the  Language  of  Experiment.  Close  the 
opening  at  .4  and  repeat  the  experiment :  the  flame  is  not  put  out. 
Remove  the  tube  and  repeat  the  blow ;  the  flame  is  not  put  out. 

The  answer  has  come.  The  tube  is  necessary  ;  whatever  blew 
out  the  candle,  did  go  through  the  tube. 

Experiment  143.— Was  this  something  a  wind  or  a  wave  ?  W« 
must  ask  our  question  in  the  same  language  as  before ;  we  must 
make  an  experiment.  Dissolve  as  much  potassium  nitrate  (salt- 
peter), as  you  can  in  half  a  cupful  of  hot  water.  Soak  a  piece  of 
unsized  paper  in  this  liquid  and  dry  it.  This  "  touch-paper  "  burns 
with  much  smoke  but  no  flame.  Burn  the  paper  in  the  tube  near 
A,  filling  that  end  of  the  tube  with  smoke.  Repeat  the  experiment 
as  before.  No  smoke  issues  at  B.  Another  answer  has  come  ; 
it  was  not  a  wind  that  passed  through  the  tube. 

Experiment  144. — Replace  the  tin  tube  by  about  the  same 
length  of  rubber  tube,  that  has  an  internal  diameter  of  from  ten 
to  fifteen  mm.  (|  inch).  Thrust  the  neck  of  a  tin  or  glass  funnel 
into  the  end  of  the  tube  at  A.  Get  a  few  inches  of  glass  tubing 
that  will  fit  snugly  into  the  rubber  tubing.  Heat  the  middle  of 
the  glass  in  a  flame  until  it  softens.  Slowly  draw  the  ends 
asunder  until  the  softened  part  is  reduced  to  a  diameter  of  about 
two  mm.  Break  the  tube  at  this  narrow  neck  and  push  the  large 
end  of  one  piece  into  the  rubber  tube  at  B.  Place  a  small  flame 
opposite  the  small  opening  of  the  glass  tube.  Strike  a  blow  in  front 
of  the  funnel  at  A  and  notice  that  a  puff  or  pulse  of  air  blows  the 
flame.  Make  a  loose  loop  in  the  rubber  tube  and  repeat  the  ex- 
periment. Clap  the  hands  at  A  and  notice  the  series  of  puffs  at  B. 
While  an  assistant  is  clapping  his  hands  at  A,  pinch  the  rubber 
tube  so  as  to  prevent  the  motion  from  passing  through  it.  Notice 
that  the  puffs  at  />  cease  while  the  tube  is  thus  pinched  and 
reappear  as  soon  as  the  tube  is  released. 

320.  Propagation  of  Sound.— Sound  is  ordinarily 
propagated  through  the  air.     The  first  layer  of  air  is 
by  the  vibrating  body,     The  particles  of  this 


§  321  WAVE  LENGTH.  249 

layer  give  their  motion  to  the  particles  of  the  next  layer, 
and  so  on  until  the  particles  of  the  last  layer  strike  upon 
the  drum  of  the  ear. 

(a.)  See  Elements  of  Natural  Philosophy,  %  484,  a.  If  the  teacher 
or  pupil  has  a  copy  of  Alfred  M.  Mayer's  little  book  on  "  Sound,"1 
he  may  well  read  the  explanation  of  the  propagation  of  sound 
given  on  p.  89.  He  will  also  do  well  to  make  and  use  Crova's 
Disk,  as  described  in  Experiment  58  of  that  book.  If  necessary,  the 
pupils  should  "club  together"  and  buy  the  book /or  the  class. 

321.  Wave  Length. — In  such  a  series  of  similar 
waves,  measuring  in  the  direction  in  which  the  waves 
are  traveling,  the  distance  from  any  vibrating  par- 
ticle to  the  next  particle  that  is  in  the  same 
relative  position  or  "  phase "  is  called  a  wave 
length.  In  the  case  of  water  waves,  for  example,  the 
horizontal  distance  from  one  crest  to  the  next  crest 
would  be  a  wave  length.  The  wave  length  may  be 
found  by  dividing  the  velocity  by  the  number  of 
vibrations. 

(a).  Every  one  knows  how  to  produce  a  series  of  waves  in  g 
rope  as  shown  in  Fig.  148,  each  curved  line  of  which  we  may 


imagine  to  be  an  instantaneous  photograph  of  a  rope  thus  shaken, 
The  distance  ub  or  cd,  is  a  half  wave  length, 


250  NATURAL  PHILOSOPHY.  §  $2$ 

222.  Amplitude. — Amplitude  means  the  distance 
between  the  extreme  positions  of  the  vibrating 
particle,  or  the  length  of  its  journey.  Referring  to 
Fig.  148,  the  distance  de  or  cf  is  the  amplitude  of  the 


Experiment  145. — Hold  one  end  of  a  straight  spring,  as  a  hick- 
ory etick,  in  a  vise,  pull  the  free  end  to  one  sido  and  let  it  go. 
Elasticity  will  return  it  to  ita 
position  of  rest,  kinetic  energy 
will  carry  it  beyond  and  so  on,  a 
vibratory  motion  toeing  thus  pro- 
duced. (Fig.  149.)  When  the 
spring  is  long,  the  vibrations 
may  be  seen.  By  lowering  the 
spring  in  the  vise,  the  vibrating 
part  is  shortened,  the  vibrations 
reduced  in  amplitude  and  in- 
creased in  rapidity.  As  the 
spring  is  shortened,  the  vibra- 
tions become  invisible  but  audi- 
ble, showing  that  a  sufficiently 
rapid  vibratory  motion  may  pro- 
duce a  sound. 
FIG.  149. 

323.  Sound  Waves. — The  layers  of  air  are  crowded 
more  closely  together  by  each  outward  vibration  of  the 
sounding  body ;  a  condensation  of  the  air  is  thus  pro- 
duced. As  the  sonorous  body  vibrates  in  the  opposite 
direction,  the  nearest  layer  of  air  particles  follow  it;  a 
rarefaction  of  the  air  is  thus  produced. 

A  sound  wave,  therefore,  consists  of  two  part& 
tt>  condensation  and  a  rarefaction/. 


§323  SOUND  WAVES.  251 


•••••••-•-u-v.-tuv.--'  •:•••  -   .-• ..-'  •'••:•/.•••::•'•••'••  •  •  -      •••:'•••.;.•//.•  .•  • 

Fw.  150. 

The  motion  of  any  air  particle  is  backward  and  for- 
ward in  the  line  of  propagation,  and  not  "  up  and  down  " 
across  that  line,  as  in  the  case  of  water  waves. 

Experiment  146. — Provide  a  wooden  rod  about  half  an  inch 
square  (or  1  inch  by  }  inch)  and  five  or  six  feet  long.  Place  one 
end  of  this  rod  (preferably  made  of  light,  dry  pine)  against  the 
panel  of  a  door,  hold  the  rod  horizontal  and  place  the  handle  of  a 
vibrating  tuning-fork  against  the  other  end.  Notice  the  sound 
given  out  by  the  panel. 

Experiment  147. — With  the  help  of  an  assistant,  vary  the  last 
experiment  by  placing  one  end  of  the  rod  against  the  ear  or  be- 
tween the  teeth  instead  of  against  the  panel.  See  if  the  sound  of 
the  fork  at  the  same  distance  is  perceptible  without  the  help  of 
the  rod. 

Experiment  148. — Make  a  "string  telephone"  as  follows: 
melt  the  bottoms  from  two  small,  round,  tin  boxes  about  an  inch 
or  an  inch  and  a  half  in  diameter.  Ground  spices  are  often  sold  in 
boxes  of  the  kind  desired.  The  bottom  may  be  removed  by  set- 
ting it  for  a  moment  on  a  hot  stove.  Over  one  end  of  each  tubo 
thus  provided,  firmly  tie  a  piece  of  well-soaked  bladder,  which 


NATURAL  PHILOSOPHY. 


$324 


may  be  had  of  a  butcher  or  apothecary.  When  the  bladder  is  dry, 
pass  one  end  of  a  long,  fine  string  through  the  middle  of  the  head 
of  each  tin  box  and  tie  a  knot  in  each  end  of  the  string  to  prevent 
it  from  drawing  back.  The  knot  should  be  on  the  inaide  of  the 
box.  Let  one  pupil  hold  the  open  end  of  one  box  to  his  ear,  the 
other  box  being  held  by  another  pupil  at  such  a  distance  that  the 


FIG.  151. 


string  shall  be  drawn  tight.  If  this  pupil  bring  the  foot  of  a 
vibrating  tuning-fork  to  the  tin,  the  vibration  will  travel  along 
the  string  and  the  sound  be  heard  by  the  first  pupil.  If  the  string 
be  a  hundred  feet  long  or  more  and  tightly  stretched,  conversa- 
tion may  be  carried  on  through  the  apparatus.  It  is  desirable 
that  no  solid  touch  the  string  between  the  tin  boxes. 

324.  Sound  Media. — Any  elastic  substance  may 
become  the  medium  for  the  transmission  of  sound, 
bat  such  a  medium  is  necessary.  Sound  cannot 
be  transmitted  through  a  vacuum. 


§  327  VELOCITY  OF  SOUND.  263 

(a.)  Soldiers  and  Indians  sometimes  detect  the  approach  of  the 
enemy  at  a  great  distance  by  putting  their  ears  to  the  ground. 

325.  Velocity  of  Sound  in  Air. — The  transmission 
of  sound  is  not  instantaneous.    The  blow  of  a  hammer  i.« 
often  seen  several  seconds  before  the  sound  is  heard ; 
steam  escaping  from  the  whistle  of  a  distant  locomotive 
becomes  visible  before  the  shrill  scream  is  audible  ;  the 
lightning  precedes  the  thunder. 

17z,<?  velocity  of  sound  in  air  at  the  freezing  tern* 
perature  is  about  332rti.,  or  1090  ft.  per  second. 

The  freezing  temperature  is  32  degrees  by  Fahren- 
heit's thermometer  or  zero  by  the  centigrade  thermom- 
eter. (§360.) 

(a.)  The  velocity  above  given  is  more  than  600  miles  an  hcur. 
A  wind  of  75  miles  an  hour  is  a  terrible  hurricane.  Fortunately, 
we  are  here  dealing  with  wave  mot.on  and  not  with  wind  motion. 

326.  Effect  of  Temperature  upon  Velocity.— 
TJiere   is  an  added   velocity   of  about   1.12  feet 
for   every  Fahrenheit   degree,  or   of  2  feet  (GO 
centimeters)  for  every  centigrade   degree   of  in- 
crease of  temperature. 

Thus,  the  velocity  of  sound  in  air  at  a  temperature  of 
59°  F.  or  15°  C.  is  about  1120  ft. 

327.  Continuous  Sound. — A  sound  may  be  momen. 
tary  or  continuous.    A  momentary  sound  consists  of  a 
single  pulse  produced  by  a  single  and  sudden  blow.     4 
continuous  sound  consists  of  a  rapid  succession  of  pulses. 


254  NATURAL  PHILOSOPHY*  $  32J 

The  ear  is  so  constructed  that  its  vibrations  disappeai 
rery  rapidly  but  the  disappearance  is  not  instantaneous. 

//  the  motion  imparted  to  the  auditory  nerve  by 
each  individual  pulse  continue  until  the  arrival 
of  its  successor,  the  sound  will  be  continuous. 

(a.)  Momentary  sounds  may  be  produced  by  pounding  with  a 
hammer,  stamping  with  the  foot,  clapping  the  hands  or  draw- 
ing a  stick  slowly  along  the  pickets  of  a  fence.  Continuous 
eounds  may  be  produced  by  sawing  boards  or  filing  saws.  They 
are  more  or  less  familiar  in  the  rattling  of  wheels  over  a  stony 
pavement,  the  roar  of  waves  or  the  crackling  of  a  large  fire. 

328.  Noise  and  Music. — The  sensation  produced 
by  a  series  of   blows  coming  at  irregular  intervals,  is 
unpleasant  and    the    sound    is    called    a  noise.      But 
when   the  air   waves  come  with  sufficient   rapidity  to 
render  the  sound  continuous  and  with  perfect  regularity, 
the  sensation  is  pleasant  and  the  sound  is  said  to  be 
musical. 

To  secure  this  pleasing  smoothness  of  music,  the 
sounding  body  must  vibrate  with  the  unerring 
regularity  of  the  pendulum,  but  impart  nuiclt 
sharper  and  quicker  shocks  to  the  air.  Every  musi- 
cal sound  has  a  well-defined  period  and  tea  re 
length. 

329.  Elements    of  Musical    Sounds.  —  Musical 
sounds  or  tones  have  three  elements — intensity  or  loud- 
ness,  pitch  and  quality. 


§  332  INTENSITY  OF  SOUND.  255 

330.  Intensity    and    Amplitude. — Loudness    of 
sound  depends  upon  the  amplitude  of  vibration. 
TJie  greater  the  amplitude,  the  louder  the  sound. 

(a.)  If  the  middle  of  a  tightly-stretched  cord  or  wire,  as  a  guitar 
string,  be  drawn  aside  from  its  position  of  rest  and  then  set  free, 
it  will  vibrate  to  and  fro  across  its  place  of  rest,  striking  the  air 
and  sending  sound  waves  to  the  ear.  If  the  middle  of  the  string  be 
drawn  aside  to  a  greater  distance  and  then  set  free,  the  swing  to 
and  fro  will  be  increased,  harder  blows  will  be  struck  upon  the 
air  and  the  air  particles  will  move  forward  and  backward  through 
a  greater  distance.  In  other  words,  the  amplitude  of  vibration 
has  been  increased  and  we  say  that  the  sound  is  louder.  (See 
Mayer's  "  Sound,"  Experiment  93.) 

Experiment  i49. — Whisper  into  one  end  of  a  length  (50  ft.)  ol 
garden  hose.  A  person  listening  with  his  ear  at  the  other  end 
of  the  hose  can  distinctly  hear  what  is  said  although  the  sound 
be  inaudible  to  a  person  holding  the  middle  of  the  hose. 

331.  Acoustic  Tubes.   -If  the  sound  ware  be  not 
allowed   to  expand  as  a  spherical  shell,   the  energy  of 
the  wave  cannot  be  diffused.     In  acoustic  tubes  (Fig. 
152),  this  diffusion  is  prevented  ;  the  waves  are  propa- 
gated in  only  one  direction.     In  this  way,  sound  may 
be  transmitted  to  great  distances  without  considerable- 
loss  of  intensity. 

Experiment  150. — Draw  the  finger  nail  across  the  teeth  of  a 
comb,  slowly  the  first  time  and  rapidly  the  second  time.  Notice 
the  difference  in  the  sounds. 

332.  Pitch. — The  second  element  of  a  musical  sound 
is  pitch,  the  quality  that  makes  the  difference  betweeu 
a  low  tone  and  a  high  tone. 


256 


NATURAL  PHILOSOPHY. 


27&e  pitch  of  a  sound  depends  upon  the  rapidity 
6f  vibration  of  the  sounding  body*  The  more 
rapid  the  vibrations,  the  higher  the  tone. 

(a.)  That  pitch  depends  upon  rapidity  of  vibration,  may  be 
shown  satisfactorily  by  means  of  Savart's 
wheel,  shown  in  Fig.  153.  This  consists 
of  a  heavy,  metal  ratchet-wheel,  supported 
on  a  frame  and  pedestal  The  wheel  may 
be  set  in  rapid  revolution  by  a  cord  wound 
around  the  axis.  By  holding  a  card  agains< 
the  teeth,  when  in  rapid  motion,  a  shriU 
tone  will  be  produced,  gradually  falling  ij 
pitch  as  the  speed  is  lessened.  (See  Mayer*8 
FIG.  153.  " Sound"  Experiments  77-80.1 


§334 


REFLECTION   OF  SOUNt). 


25? 


333.  Reflection  of  Sound. — When  a  sound  ray  strikes 
an  obstacle,  it  is  reflected  in  obedience  to  the  principle 
given  in  §  57. 

(a.)  "  The  great  dome  of  St.  Paul's  Cathedral  in  London  is  so 
constructed  that  two  persons  at  opposite  points  of  the  internal 
gallery,  placed  in  the  drum  of  the  dome,  can  talk  together  in  a 
mere  whisper.  The  sound  is  transmitted  from  one  to  the  other 
by  successive  reflections  along  the  course  of  the  dome." 

A  similar  phenomenon  is  observable  in  the  dome  of  the  Capitol 
at  Washington.  The  "guides"  about  the  building  will  point  out 
for  you  the  proper  position  in  the  gallery  of  the  dome  and  also  cer- 
tain places  on  the  floor  of  Statuary  Hall  (formerly  the  Hall  of 
Representatives),  where  remarkable  acoustic  phenomena  may  be 
noticed. 

334.  Recapitulation.— To  be  amplified  by  the  pupil 
for  review. 


SOUND. 


DEFINITION. 
CAUSE. 
WAVES 

MEDIA. 

VELOCITY 

MOMENTARY. 

CONTINUOUS... 


REFLECTION... 


MODE  -JF  PROPAGATION. 
LENGI  ... 
AMPLITUDE. 

(  Condensation. 

PARTS < 

(  Rarefaction. 

AT  FREEZING  TEMPERATURE. 
AT  OTHER  TEMPERATURES. 
NOISY. 

Loud. 


MUSICAL.. 


(  Cause. 

(  Acoustic  Tubes 

Pitch Cause. 

Quality. 


I  WHIS 


ISPERING  GALLERIES 


NATURAL  PHILOSOPHY.  §  334 


EXERCISES. 

1.  Make  a  pencil  sketch  of  an  "  up-and-down "  wave  having 
a  length  of  two  inches  and  an  amplitude  of  half  an  inch. 

2.  Water  is  just  beginning" to  freeze  in  the  open  air.    What  U 
the  velocity  of  sound  ? 

3.  If  a  tuning  fork  vibrates  256  times  a  second  and  its  sound 
travels  1280  feet  in  a  second,  what  is  the  wave  length  ? 

4.  The  thermometer  records  a  freezing  temperature.     In  a  sec- 
ond,  218  sound  waves  pass  a  given  point.     What  is  the  length  of 
each  wave?  Ans.     5  feet. 

5.  What  is  the  rate  of  vibrations  of  a  body  that  produces  sound 
waves  just  a  meter  long  when  it  is  just  freezing  cold  ? 

6.  What  is  the  velocity  of  sound  in  air  at  a  temperature  of 
30°  G?  Am.    1130  feet. 

7.  When  sound  has  a  velocity  of  1126  feet  per  second,  what  is 
the  temperature?  An*.    It  is  18°  C. 

8.  Give  the  definition  anf*  correct  pronunciation  of  the  word 


SECTION     II. 

THE  TELEPHONE— COMPOSITION  AND  ANALYSIS  OF 
SOUNDS. 

335.  The  Telephone. — This  instrument  is  repre- 
sented in  section  by  Fig.  154.  A  is  a  permanent  bar 
magnet,  around  one  end  of  which  is  wound  a  coil,  E,  of 
fine  copper  wire  carefully  insulated.  The  ends  of  this 
coiled  wire  are  attached  to  the  larger  wires,  CCy  which 


FIG.  154. 

communicate  with  the  binding  posts,  DD.  In  front  oi 
the  magnet  and  coil  is  the  soft  iron  diaphragm,  E>  which 
corresponds  to  the  disc,  a,  of  Fig  144. 

In  front  of  the  diaphragm  is  a  wooden  mouth-piece 
with  a  hole,  about  the  size  of  a  dime,  at  the  middle  of 
the  diaphragm  and  opposite  the  end  of  the  magnet. 
The  outer  case  is  made  of  wood  or  of  hard  rubber.  The 
external  appearance  of  the  complete  instrument  is  repre- 
sented by  Fig.  155.  The  binding  posts  of  one  instru- 


300 


NATURAL  PHILOSOPHY. 


§337 


toent  being  connected  by  wires  with  the  binding  posta 
rf  another  at  a  distance,  conversation  may  be  carried  on 

between  them.     Carefully  review  §§ 

314  and  315. 

336.  Action  of  the  Tele- 
phone.— When  the  mouth-piece  is 
brought  before  the  lips  of  a  person 
who  is  talking,  air  waves  beat  upon 
the  diaphragm  and  cause  it  to  vi- 
brate. Eacli  vibration  of  the  dia- 
phragm induces  an  electric  current 
in  the  wire  of  B.  These  curreu  ts  are 
transmitted  to  the  coil  of  the  con- 
nected telephone,  at  a  distance  of, 
perhaps,  several  miles,  and  there 
produce  vibrations  exactly  like  the 
original  vibrations  produced  by  the 
voice  of  the  speaker.  These  vibra- 
tions of  the  second  diaphragm  send  out  new  air  waves. 
The  two  sets  of  air  waves  being  alike,  the  resulting  sen- 
sations produced  in  the  hearers  are  alike.  Not  only  dif- 
ferent words  but  also  different  voices  may  be  recognized. 
Remember  that  an  electric  current,  and  not  sound  waves, 
passes  along  the  line  wire.  The  arrangement  being  the 
same  at  both  stations,  the  apparatus  works  in  either  di- 
rection. No  battery  is  necessary  with  this  arrangement 

337.  The  Transmitter. — In  practice,  a  transmitter, 
shown  at  C  in  Fig.  156  is  generally  used.  The  vibrations 
of  the  diaphragm  of  C,  when  acted  upon  by  sound  waves, 


.  155. 


§337 


THE  TELEPHONE. 


produce  a  varying  pressure  upon  a  carbon  button  placed  in 
the  circuit  of  a  galvanic  battery,  D.  This  varying  pnr  > 
ure  results  in  a  varying  resistance  to  the  passage  of  the 


Fio.  156. 

current  through  the  button  and,  consequently,  in  varia 
tions  in  the  current  itself.  This  varying  current,  passing 
through  the  primary  circuit  of  a  small  induction  coil  ir. 
the  box,  C,  induces  a  current  in  the  secondary  IMOUII 
thereof.  This  current,  thus  induced,  flows  over  the  L  .e 
phone  wires  and,  at  the  other  station,  passes  thro  igii  a 
telephone  like  that  shown  at  B3  which  is  held  close  If 
the  ear  of  the  listener. 


262  NATURAL   PHILOSOPHY.  §337 

The  message  is  transmitted  by  C  at  one  station  and 
received  by  B,  of  a  similar  instrument  at  the  other  station. 

At  each  station  is  placed  an  electric  bell,  A,  which  may 
be  rung  from  the  other  station,  for  the  purpose  of  at- 
tracting attention.  When  the  stations  are  a  considerable 
distance,  apart,  one  binding  post  of  each  instrument 
may  be  connected  with  the  earth,  as  in  the  case  of  the 
telegraph.  See  Fig.  15G. 

(a.)  In  most  of  our  cities,  the  telephones  are  connected  by  wire 
with  a  central  station,  called  a  telephone  exchange.  The  "  Ex- 
change" may  thus  be  connected  with  the  houses  of  hundreds  of 
patrons  in  all  parts  of  the  city  or  even  in  different  cities.  Upon  re- 
quest by  telephone,  the  attendant  at  the  central  station  connects 
the  line  from  any  instrument  with  that  running  to  any  other  in- 
strument. Thus,  each  subscriber  may  communicate  directly  with 
any  other  subscriber  to  the  exchange. 

Experiment  151. — The  effect  of  repeated  impulses,  each  feeble 
but  acting  at  the  right  instant,  may  be  forcibly  illustrated  as  follows: 
Support  a  heavy  weight,  as  a  bucket  of  coal,  by  a  long  string  or 
wire.  To  the  handle  of  a  bucket,  fasten  a  fine  cotton  thread.  By 
repeated  pulls  upon  the  thread,  each  pull  after  the  first  one  being 
given  just  as  the  pendulum  is  beginning  to  swing  toward  yo* 
from  the  effect  of  the  previous  pull,  the  weight  may  be  made  to 
swing  through  a  large  arc,  while  a  single  pull  out  of  time  will 
snap  the  thread.  A  little  practice  will  enable  you  to  perform  the 
experiment  neatly. 

Experiment  152. — Vary  the  last  experiment  by  setting  the 
pendulum  in  motion  by  well-timed  puffs  of  air  from  the  mouth  or 
from  a  hand  bellows. 

The  same  principle  is  illustrated  in  the  action  of  the  spring 
board,  familiar  to  most  boys,  who  know  that  the  desired  effect 
san  be  secured  only  by  "keeping  time."  Soldiers  are  often  or 


§337 


SYMPATHETIC    VIBRATIONS. 


263 


deb 


dered  to  "  break  step  "  in  crossing  a  bridge,  lest  the  accumulated 
energy  of  many  footfalls  in  unison  break  the  bridge. 

Experiment  153.— Suspend  several  pendulums  from  a  frame  as 
shown  in  Fig.  157.  Make  two  of  equal  length  so  that  they  will  vi- 
brate at  the  same  rate.  Be  sure  that  tliey  will  thus  vibrate.  The 
other  pendulums  are  to  be  of  different  lengths.  Set  a  in  vibration. 
The  swinging  of  a  will  produce  slight  vibrations  in  the  frame 
which  will,  in  turn,  transmit  them  to  the  other 
pendulums.  As  the  successive  impulses  thus 
imparted  by  a  keep  time  with  the  vibrations 
of  b,  this  energy  accumulates  in  b,  which  is 
soon  set  in  perceptible  vibration. 

As  these  impulses  do  not  keep  time  with  the 
vibrations  of  the  other  pendulums,  there  can 
be  no  such  accumulation  of  energy  in  them,  for 
many  of  the  impulses  will  act  in  opposition  to 
the  motions  produced  by  previous  impulses  and 
lend  to  destroy  them. 

Experiment  154.— Place  two  mounted  tun- 
ing forks  (Fig.  159)  several  feet  apart.     The 
forks  must  be  exactly  in  unison.    Sound  one  j 
fork  by  rapidly  separating  its  prongs  with  a 
wooden  rod  or  by  drawing  a  resined  bass-viol 
bow  across  their  ends.     After  a  few  seconds, 
stop  the  vibrations  of  this  fork  with  the  fin- 
gers  ;  it  will  be  found  that  the  other  fork  has 
been  put  into  sympathetic  vibration  and  is  giving  forth  a  souno. 
Weight  one  of  the  prongs  of  the  second  fork  with  wax ;  an  at- 
'.empt  to  repeat  the  experiment  will  fail. 

Experiment  155.— Tune,  to  unison,  two  strings  upon  the  same 
eonometer  (Fig.  158).  Upon  one  string,  place  two  or  three  paper 
riders.  With  a  violin  bow,  set  the  other  string  in  vibration.  The 
sympathetic  vibrations  thus  produced  will  be  shown  by  the 


264 


NATURAL  PHILOSOPHY. 


§338 


dismounting  of  the  riders,  whether  the  vibrations  be  audible  or 
not 

Change  the  tension  of  one  of  the  strings,  thus  destroying  the 
unison.  Repeat  the  experiment  and  notice  that  the  sympathetic 
vibrations  are  not  produced. 

338.  Sympathetic  Vibrations.— The  string  of  a 
violin  may  be  made  to  vibrate  audibly  by  sounding  near 
it  a  tuning-fork  of  tbe  same  tone.  By  prolonging  a 
vocal  tone  near  a  piano,  one  of  the  wires  seems  to  take 
up  the  note  and  give  it  back  of  its  own  accord.  If 
the  tone  be  changed,  another  wire  will  give  it  back. 
Thus  the  vibrations  of  the  strings  may  produce  sonorous 
waves  and  the  waves,  in  turn,  may  produce  vibrations 
in  another  string. 

The  string  absorbs  only  the  particular  kind  of 
vibration  that  it  is  capable  of  producing. 


PIG.  158. 

(a.)  The  sonometer  box  may  be  made  by  any  carpenter.  It  is 
about  fifty-nine  inches  long,  4f  inches  wide  and  4f  inches  deep. 
The  ends  are  made  of  inch  oak  boards,  the  sides  of  ^  inch  oak 
boards  and  the  top  of  £  inch  pine  board.  The  top  should  be  glued 


§338  SYMPATHETIC    VIBRATIONS.  265 

on  ;  no  bottom  is  needed ;  the  box  may  sit  directly  on  the  table 
Three  or  four  one-inch  holes  may  wel)  be  bored  in  each  side-piece 
The  two  bridges,  shown  at  A  and  B  (Fig.  158)  should  be  of  very 
hard  wood  and  glued  to  the  cover  just  47f  inchec  (120  centimeters) 
apart,  measured  from  centre  to  ceutre.  The  strings  may  be  such 
as  are  used  on  bass-viols ;  they  should  be  alike.  Two  similar 
pieces  of  piano-forte  wire  (large  size)  may  be  used.  The  strings 
may  be  stretched  by  weights  as  shown  in  the  figure  or  by  two  piano 
string  pegs  turned  with  a  wrench  or  a  piano  tuner's  key.  The 
familiar  screw  arrangement  of  the  bass-viol  may  be  used  for  the 
purpose.  If  piano  wires  are  used  for  strings,  the  ends  must  be 
annealed  by  heating  them  red  hot  and  cooling  them  slowly,  BO 
that  they  may  remain  fixed  when  wound  around  their  fasten 
ings.  Lines  should  be  drawn  across  the  top  of  the  box,  exactly 
dividing  the  distance  between  the  middle  of  the  bridges  (at  which 
points  the  strings  are  supported)  into  halves,  thirds  and  quarters. 
Provide  a  block  of  wood,  about  two  inches  wide,  4£  inches  long 
and  just  thick  enough  to  slip  between  the  strings  and  the  top  of 
the  box. 

(6.)  When  the  two  strings  are  in  unison,  they  will  vibrate  at 
exactly  the  same  rate.  The  second  and  subsequent  pulses  sent  out 
by  the  first  string  strike  the  second  string,  already  vibrating  from 
the  effect  of  the  first  pulse,  in  the  same  phase  of  vibration,  and 
thus  each  adds  its  effect  to  that  of  all  its  predecessors.  If  the 
strings  be  not  in  unison,  they  will  vibrate  at  different  rates  and  but 
few  of  the  successive  pulses  can  strike  the  second  string  in  the 
same  phase  of  vibration  ;  the  greater  number  will  strike  it  at  the 
wrong  instant. 

Experiment  156.— Strike  a  tuning-fork  held  in  the  hand. 
Notice  that  the  sound  heard  is  feeble.  Strike  the  fork  again  and 
place  the  end  of  the  handle  upon  a  table.  The  loudness  of  the 
sound  heard  is  remarkably  increased.  Repeat  Experiment  146. 

Experiment    157.— Strike  the  fork  and  hold  it  near  the  eat 


NATURAL  PHILOSOPHY. 


§339 


counting  the  number  of  seconds  that  you  can  hear  it:  Strike  the 
fork  again  with  equal  force,  place  the  end  of  the  handle  on  the 
table  and  count  the  number  of  seconds  that  you  can  hear  it. 

339.  Sounding-boards. — In  the  case  of  the  sonom- 
eter, piano,  violin,  guitar,  etc.,  the  sound  is  due  more 
to  the  vibrations  of  the  resonant  bodies  that  carry  the 
strings  than  to  the  vibrations  of  the  strings  themselves. 
The  strings  are  too  thin  to  impart  enough  motion  to  the 
air  to  be  sensible  at  any  considerable  distance ;  but  as 
they  vibrate,  their  tremors  are  carried  by  the  bridges  to 
the  material  of  the  sounding  apparatus  with  which  they 
are  connected. 

These  larger  surfaces  throw  larger 
masses  of  air  into  vibration  and  thus 
greatly  intensify  their  sound.  It 
necessarily  follows  that  the  energy 
of  the  vibrating  body  is  sooner  ex- 
hausted ;  the  sounds  are  of  shorter 
*  duration. 

FIG.  159.  (<*•)  For  class  or  lecture  experiments, 

tuning  forks  should  be  mounted  as  shown 
in  Fig.  159. 

Experiment  158.— Support  horizontally,  between  two  fixed  sup- 
ports, a  soft  cotton  rope  a  few  yards  in  length.  With  a  stick, 
Strike  the  rope  near  one  end  a  blow  from  below  and  a  crest  will  be 


FIG.  160. 


§  34O  COINCIDENT  WA  VES.  267 

formed  as  shown  in  Fig.  160.  Vary  the  tension  of  the  rope,  if 
necessary,  until  the  crest  is  easily  seen.  Notice  that  the  crest,  c, 
travels  from  A  to  B  where  it  is  reflected  back  to  A  as  a  trough,*. 
By  striking  the  rope  from  above,  a  trough  may  be  started  which 
will  be  reflected  as  a  crest.  See  Mayer's  "Sound,"  Experiment 
57,  in  which  tho  wire  spring  shows  waves  of  condensation  and 
rarefaction.  Tie  a  piece  of  string  to  one  turn  of  the  wire  and  no- 
tice the  motion  of  the  string  as  the  wave  passes. 

Experiment  159.— Start  from  A  a  trough.  At  the  moment  of  its 
reflection  as  a  crest  at  B,  start  a  crest  at  A  as  shown  in  Fig.  161. 
The  two  crests  will  meet  near  the  middle  of  the  rope.  The  crest 
at  the  point  and  moment  of  meeting  results  from  two  forces  acting 


FIG.  161. 

in  the  same  direction  consequently,  it  will  be  grea^r  than  either 
of  the  component  crests. 

340.  Coincident  Waves. — In  the  case  of  water 
waves,  when  crest  coincides  with  crest,  the  water  reaches 
a  greater  height.  So  with  sound  waves,  when,  conden- 
sation coincides  with  condensation,  this  part  of 
the  wave  will  be  more  condensed;  when  rarefac- 
tion coincides  with  rarefaction,  this  part  of  the 
wave  will  be  more  r  art  fled. 

This  increased  difference  of  density  in  the  two  parts 
of  the  wave  means  increased  loudness  of  sound,  because 
there  is  an  increased  amplitude  of  vibration  for  the  par- 
ticles constituting  the  wave. 


268  NATURAL  PHILOSOPHY.  §  341 

341.  Reinforcement  of  Sound. — This  increased 
intensity  may  result  from  the  blending  of  two  or  more 
series  of  similar  waves  in  like  phases,  or  from  the  union 
of  direct  or  reflected  waves  in  like  phases. 

Under  such  circumstances,  one  set  of  waves  is 
said  to  reinforce  the  other.  The  pJienomenon  is 
spoken  of  as  the  reinforcement  of  sound. 

Experiment  160.— Hold  a  sounding  tuning-fork  over  the  mouth 

of  a  glass  jar,  18  or  20 
inches  deep  ;  a  feeble 
sound  is  heard.  Care- 
fully pour  in  water  until 
the  liquid  reaches  such 
a  level  that  the  sound 
suddenly  becomes  much 
louder.  The  water  has 
shortened  the  air  col- 
umn until  it  is  able  to 
vibrate  in  unison  with 
the  fork.  The  length 
of  the  air  column  is  one- 
fourth  the  length  of  the 
wave  produced  by  the 
fork. 

342.  Resonance. 
— Resonance  is  a 
variety  of  the  re- 
inforcement  of 
sound  due  to 
sympathetic  vibrations.  The  resonant  effects  of 
solids  were  shown  in  339.  The  resonance  of  an  ail 


g  343  RESONANCE.  269 

column  was  shown  in  the  last  experiment.  The  loudness 
of  sound  of  wind  instruments,  like  the  pipes  of  a  church 
organ,  is  largely  due  to  the  resonance  of  the  air  enclosed 
in  them. 

343.  Helmholtz's  Resonators. — Helmholtz,  the 
German  physicist,  constructed  a  series  of  resonators,  each 
one  of  which  resounds  powerfully  to  a  single  tone  of 
certain  pitch  or  wave  length.  They  are  metallic  vessels, 
nearly  spherical,  hav- 
ing a  large  opening, 
as  at  A  in  Fig.  163, 
for  the  admission  of 
the  sound  waves.  The 
funnel-shaped  projec- 
tion at  B  has  a  small 
opening  and  is  inserted 
in  the  outer  ear  of  the 
observer.  Fm 

Experiment  161.— Using  the  rope  as  described  in  Experiment 
158,  start  a  crest  at  A.  At  the  moment  of  its  reflection  at  B  as  a 
trough,  start  a  second  crest  at  A.  The  trough  and  crest  will  meet 
near  tlie  middle  of  the  rope.  The  rope  at  this  time  and  place  will 
be  urged  upward  by  the  crest  and  downward  by  the  trough.  The 


FIG.   164. 


resultan*  effect  of  these  opposing  forces  will,  of  course,  be  equal 
to  their  difference.  If  crest  and  trough  exert  equal  forces,  the 
difference  will  be  zero.  Consequently  the  motion  of  the  rope  at 


270  NATURAL  PHILOSOPHY.  §  345 

the  meeting  of  crest  and  trough  will  be  little  or  nothing.  Thu» 
one  wave  motion  may  be  made  to  destroy  the  effect  of 
another  wave  motion. 

Experiment  162.— Hold  a  vibrating  tuning-fork  near  the  eat 
and  slowly  turn  it  between  the  fingers.  During  a  single  complete 
rotation,  four  positions  of  full  sourd  and  four  positions  of 
perfect  silence  will  be  found.  When  a  side  of  the  fork  is  parallel 


FIG.   165. 

to  the  ear,  the  sound  is  plainly  audible  ,  when  a  comer  of  the 
prong  is  turned  toward  the  ear,  the  waves  from  one  prong  com- 
pletely destroy  the  waves  started  by  the  other. 

Experiment  163.— Over  a  resonant  jar,  as  shown  in  Pig.  164, 
slowly  turn  a  vibrating  tuning-fork.  In  four  positions  of  the  fork 
we  have  loud,  resonant  tones  ;  in  four  other  positions  we  have 
complete  interference.  While  the  fork  is  in  one  of  these  positions 


§  344  INTERFERENCE   OF  SOUND.  271 

of  interference,  place  a  pastoboard  tube  around  one  of  the  vibrating 
prongs,  Fig.  167;  a  resonant  tone  is  instantly  heard;  the  cause 
of  the  interference  has  been  removed.  (See  Mayer's  "Sound,' 
Experiments  60  to  67.) 

344.  Interference  of  Sound.— If,  while  a  tuning, 
fork  is  vibrating,  a  second  fork  be  set  in  vibration,  the 
waves  from  the  second  must  traverse  the  air  set  in  mo- 
tion by  the  former.  If  the  waves  from  the  two  forks  be 
of  equal  length,  as  will  be  the  case  when  the  two  forks 
have  the  same  pitch,  and  the  forks  be  any  number  of 
whole  wave  lengths  apart,  the  two  sets  of  waves  will 
unite  in  like  phases  (Fig.  1G6),  condensation  with  con- 
densation, etc.,  and  a  reinforcement  of  sound  will  ensue. 


PIG.  166. 


But  if  the  second  fork  be  placed  an  odd  number  of 
half  wave  lengths  behind  the  other,  the  two  series  of 
waves  will  meet  in  opposite  phases ;  where  the  first  fork 
requires  a  condensation,  the  second  will  require  a  rarefac- 
tion. The  two  sets  of  waves  will  interfere,  the  one  with 
the  other.  If  the  waves  be  of  equal  intensity,  the  air 
particles,  thus  acted  upon,  will  remain  at  rest;  this  means 
silence.  In  Fig.  167,  an  attempt  is  made  to  represent 
this  effect  to  the  eye,  the  uniformity  of  tint  indicating 
the  absence  of  condensations  and  rarefactions. 


272  NATURAL  PffTLOSOPffT.  §345 


FlG.  167. 


By  adding  sound  to  sound,  both  may  be  de- 
stroyed. This  is  the  leading,  characteristic  prop- 
erty of  wave  rnotion.  The  phenomenon  here  de- 
scribed is  called  interference  of  sound. 

(a.)  The  sound  of  a  vibrating  tuning-fork  held  in  the  hand 
is  almost  inaudible.  The  feebleness  results  largely  from  inter- 
ference. As  the  prougs  always  vibrate  in  opposite  directions  at 
Ihe  same  time,  one  demands  a  rarefaction  where  the  other  de- 
mands a  condensation.  By  covering  one  vi orating  prong  with  a 
pasteboard  tube,  the  sound  is  more  easily  heard. 

Experiment  164. — Use  the  two  forks  mentioned  in  Experiment 
154,  one  of  them  being  loaded  with  wax.  Set  both  forks  in  vibra- 
tion and  notice  the  palpitating  effect. 

Experiment  165. —In  a  quiet  room,  strike  simultaneously  one 
of  the  lower  white  keys  of  a  piano  and  the  adjoining  black  key. 
The  palpitating  effect  will  be  heard. 

345.  Beats. — If  two  tuning-forks,  A  and  B,  vibrating 
respectively  255  and  256  times  a  second,  be  set  in 
vibration  at  the  same  time,  their  first  waves  will  meet  in 
like  phases  and  the  result  will  be  an  intensity  of  sound 
greater  than  that  of  either.  After  half  a  second,  B 
having  gained  half  a  vibration  upon  A,  the  waves  will 
meet  in  opposite  phases  and  the  sound  will  bo  weakened 


§  347  VIBRATIONS   OF  STRINGS.  273 

DF  destroyed.  At  the  end  of  the  second,  we  shall  have 
another  reinforcement ;  at  the  middle  of  the  next  second, 
another  interference. 

This  peculiar  palpitating  effect  is  due  to  a  suc- 
cession of  reinforcements  and  interferences,  and  is 
called  a  beat. 

The  number  of  beats  per  second  equals  the  differ- 
ence of  the  two  numbers  of  vibrations.  (See  Mayer's 
"  Sound,"  Experiments  71  and  72. 

346.  Vibrations  of  Strings.— The  laws  of  musical 
tones  are  best  studied  with  the  help  of  stringed  instru- 
ments.   The  Laws  of  the  Vibrations  of  Strings  is  treated 
in  §  455  of  the  Elements  of  Natural  Philosophy. 

Experiment  166. — Give  a  fixed  support  to  one  end  of  a  rubber 
tube  about  two  yards  long  and  hold  the  other  end  in  the  hand. 
Move  the  hand  up  and  down  so  as  to  send  a  series  of  waves  along 
the  tube.  By  varying  the  tension  of  the  tube  and  the  rate  of  vi- 
bration of  the  hand,  you  will  soon  be  able  to  produce  an  appear- 
ance of  gauzy  spindles  much  like  those  shown  in  Figs.  148  and 
170.  This  gauzy  appearance  is  due  to  an  optical  effect  known  as 
"  the  persistence  of  vision."  The  motion  of  the  hand  produces  a 
regular  succession  of  equal  crests  and  troughs,  which  are  reflected 
and  neutralize  each  other  as  explained  in  Experiment  161. 

347.  Fundamental   Tones   and  Overtones.— A 
string  may  vibrate  transversely  as  a  whole,  or  as  inde- 
pendent segments.     Such  segments  will  be  aliquot  parts 
of  the  whole  string  and  separated  from  each  other  by 
points  of  no  motion  called  nodes  or  nodal  points.     (See 
a  and  b,  Fig.  148.)     The  part  of  the  string  between  tw« 
adjacent  nodes  is  called  a  ventral  segment. 


2?4  NATURAL  PHILOSOPHY.  §  347 

The  tone  produced  by  the  vibrations  of  ili6 
whole  length  of  a  string  is  called  its  fundamental 
tone.  The  tones  produced  by  the  vibrations  of 
the  segments  of  a  string  are  called  its  overtones  or 
harmonics. 

348.  Fundamental  Tones. — When  a  string  vibrates 
so  as  to  produce  its  fundamental  tone,  its  extreme  po- 
sitions may  be  represented 
by  the  continuous  and  the 

M8-  dotted  lines  of  Fig.  1C8. 

This  effect  is  obtained  by  leaving  the  string  free  and 
bowing  it  near  one  of  its  ends. 

If  a  number  of  little  strips  of  paper,  doubled  in  the 
middle,  be  placed  like  riders  upon  the  string  and  the 
string  bowed  as  just  described,  all  of  the  riders  will  be 
thrown  up  and  most  of  them  off.  This  shows  that  the 
whole  string  vibrates  as  one  string ;  that  there  is  no  part 
of  it  between  the  fixed  ends  that  is  not  in  vibration. 

349.  The  First  Overtone. — If  the  string  of  the 
sonometer  be  touched  exactly  at  its  middle  with  a  feather, 
a   higher  tone   is  pro- 
duced when  che  string  is     ^--- —       IZx^ — HI =» 

howod     This  tone  is  an  ^  ^ ; 

octave  higher  than  the 

fundamental.     The  string  now  vibrates  in  such  a  way 

that  the  point  touched  remains  at  rest;  it  is  a  node. 

The  tone  is  due  to  the  vibrations  of  the  tuo 
Halves  of  the  string,  which  thus  give  the  octave 
instead  of  the  fundamental.  The  existence  of  the 


§350  orsnTONES.  275 

node  and  segments  will  continue  for  some  time  after  the 
feather  is  removed.  If  riders  be  placed  at  C,  JVand  D, 
the  one  at  N  will  remain  at  rest  while  those  at  0  and 
D  will  probably  be  dismounted. 

(a.)  Instead  of  the  sonometer  string,  a  piece  of  rubber  tubing 
filled  with  sand  and  hung  in  a  vertical  position  with  both  ends 
fastened  may  be  used.  Hold  the  middle  of  the  tube  to  form  a 
node  and  pluck  at  the  middle  of  the  lower  half. 


FIG.  170. 


350.  Higher  Overtones.— In  like  manner,  if  the 
vibrating  string  be  touched  at  exactly  one-third  (see 
L'ig.  J  70),  one-fourth  or  one-fifth  of  its  length  from  one 
end,  it  will  divide  into  three,  four  or  live  segments,  with 


276 


NATURAL   PHILOSOPHY. 


§350 


vil:  rations  three,  four  or  five  times  as  rapid  as  the  fun- 
damental vibrations. 

351.  Quality  or  Timbre. — As  a  sounding  body  vi- 
brates as  a  whole  and  in  segments  at  the  same  timCj 
the  fundamental  and  the  harmonics  blend.  The  result- 
ant effect  of  this  blending  of  fundamentals  and 
harmonics  constitutes  what  we  call  the  quality 
or  timbre  of  the  sound. 

We  recognize  the  voice  of  a  friend  not  by  its  loudness 
not  by  its  pitch,  but  by  its  quality.  When  a  piano  and 
violin  sound  the  same  tone,  we  easily  distinguish  the 
Bouivd  Nf  one  from  that  of  the  other  because,  while  the 
fundamentals  are  alike,  the  harmonics  are  different. 

Experiment  167. — Take  your  seat  before  the  key-board  of  a 
piacd.  Press  and  hold  down  the  key  of  "middle  C,"  marked  1  in 
Fi^  171  which  represents  part  of  the  key -board.  This  will  lift  the 


FIG.  171. 

damper  from  the  corresponding  piano  wire  and  leave  it  free  to  vi- 
brato. Strongly  strike  the  key  of  0",  an  octave  below.  Held  this 
key  down  for  a  few  seconds  and  then  remove  the  finger  The 
damper  will  fall  upon  the  vibrating  wire  and  bring  it  to  rest.  Whoa 
the  sound  of  0'  has  died  away,  a  sound  of  higher  pitch  is  heard. 
The  tone  corresponds  to  the  wire  of  1,  which  wire  is  now  vibrating 
These  vibrations  are  sympathetic  with  those  that  produced  the 
first  overtones  of  the  wire  that  was  struck.  These  vibrations  in 


§  352  COMPOUND  TOXES.  277 

the  wire  of  1  prove  the  presence  of  the  first  overtone  In  the 
vibrating  wire  of  C'.  (See  §  338.) 

In  similar  manner,  successively  raise  the  dampers  from  the  wires 
of  2,  3, 4,  5,  6  and  7,  striking  C"  each  time.  These  wires  will  ac 
cumulate  the  energy  of  the  waves  that  correspond  to  the  respec 
tive  overtones  of  the  wire  of  G*  and  give  forth  each  its  proper  tone 
Thus  we  analyze  the  sound  of  the  wire  of  C'  and  prove  that 
at  least  seven  overtones  are  blended  with  its  fundamental. 

Some  of  these  tones  of  higher  pitch,  thus  produced  by  VIM  aliens 
sympathetic  with  the  vi'srcUions  of  the  segments  of  the  wire  o)'  «7 .,  »r»- 
feebler  than  others.  This  shows  that  the  quality  of  a  f  on»  vie 
pends  upon  the  relative  intensities  as  well  as  the  numbei  0  Hut. 
overtones  that  blend  with  the  fundamental. 

352.  Simple  and  Compound   Tones. — The  weP. 

trained  ear  can  detect  several  sounds  of  different  pitch 
when  a  single  key  of  a  piano  is  struck.  In  other  wwiiia. 
the  sound  of  a  vibrating  piano  wire  is  a  compound  sound. 
The  sound  of  a  timing  fork  is  a  fairly  good  example  of  a 
simple  sound.  Simple  sounds  all  have  the  same  quality, 
differing  only  in  loudness  and  pitch. 

(a.)  A  series  of  Helmholtz's  resonators  enables  the  student  of 
acoustics  to  analyze  any  compound  sound.  Each  component  tone 
may  be  reproduced  by  a  tuning  fork  of  appropriate  pitch.  By 
sounding  simultaneously  the  necessary  number  of  forks,  each  of 
proper  pitch  and  with  appropriate  relative  intensity,  Hehnholtz 
showed  that  the  sounds  of  musical  instruments,  including  evei  the 
most  wonderful  one  of  all  (the  human  voice),  may  be  produced 
synthetically. 

NOTE.— The  author  takes  this  last  opportunity  to  advise  the 
pupil  to  procure  and  study  Mayer's  "  Sound."  See  Chapter  5VH 


278 


NATURAL  PHILOSOPHY. 


353 


353.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


1 


SYMPATHETIC 
VIBRATIONS. 


ANALYSIS  OF 


f  CONSTRUCTION. 


Resonance. 


SONOMETER  

MOUNTED  TUNING-FORKS.  . . 

SOUNDING  BOARDS 

AIR  COLUMNS. 

HELMHOLTZ'S  RESONATORS.. 
REINFORCEMENT 

f  Characteristic  of  wave  motion 
INTERFERENCE...  \ 

[Seats. 

FUNDAMENTAL  TONES. 
OVERTONES. 


§  353  EXERCISES.  279 

EXERCISES. 

1.  Two  forks  vibrate  256  and  264  times  a  second  respectively. 
(«.)  Which  yields  the  longer  soundwaves?    Which  produces 

tones  of  the  higher  pitch  ? 

2.  What  is  the  length  of  waves  produced  by  a  fork  vibrating 
280  times  a  second,  when  a  centigrade  thermometer  shows  a  tem- 
perature of  15  degrees  ?  Ana.    4  feet. 

3.  A  resonant  air  column  (Fig.  162)  is  one  foot  long.     What  is 
the  wave  length  of  the  tone  to  which  it  will  respond  most  satis- 
factorily ?  Ana.    4  feet. 

4.  One  tuning-fork  vibrates  256  times  a  second  and  makes  two 
beats  per  second  with  a  second  fork.     What  is  the  rate  of  vibra- 
tion of  the  second  fork  ? 

5.  Is  the  motion  of  a  sound  wave  vibratory  or  progressive? 

6.  What  is  the  difference  between  a  longitudinal  and  a  traaa 
verse  vibration  ?    Illustrate  each. 

7.  What  is  the  velocity  of  sound  in  a  vacuum? 

8.  What  determines  the  pitch  of  a  musical  tone* 


NATURAL  PHILOSOPHY. 


353 


REVIEW     QUESTIONS. 

1.  (a.)  How  may  one  vary  the  pitch  of  a  violin  string?    (6.) 
Describe  the  electrical  condition  of  a  polarized  body. 

2.  When  a  door  is  swung  open  or  shut  by  pressing  with  the 
hand  near  the  hinges,  what  class  of  levers  is  illustrated  ? 

3.  Fig.  172  represents  apparatus  made  of  glass  tubing  and  a 
bottle.     There  are  openings  at  a,  b  and  c.     Tell  how  the  apparatus 

a  may  be  used  to  transfer  a  liquid  that  is  dangerous  to 

handle,  e.  g.  sulphuric  acid. 

4.  Describe  the  waves  produced  by  throwing  a  stone 
into  quiet  water. 

5.  Describe  the  waves  produced  by  striking  a  bell 
in  the  air. 

6.  What  is  the  difference  between  kinetic  and  po- 
tential energy. 

7.  Define  physical  change  and  give  an  illustration 
thereof. 

8.  Describe  the  barometet 

9.  What  is  always  the  divisor  in  problems  in  spe- 
cific gravity? 

10.  WhatisaKathion? 

11.  What  is  meant  by  the  length  of  a  sound  wave  ? 
On  a  certain  dav  at  the  freezin&  temperature,  I 

saw  the  flash  of  a  gun.  Ten  seconds  later,  I  heard  the 
report,  (a.)  What  was  the  distance  of  the  gun?  (b.)  Why  do  I 
state  the  temperature  ? 


FIG    172 

' 


FIG.  173. 


13.  Fig.  173  represents  an  electric  current  flowing  through  an 
insulated  wire  wound  around  a  piece  of  soft  iron.  What  is  the 
name  of  this  combination? 


CHAPTEB    ¥UI* 
HEAT. 


SECTION     I. 

TEMPERATURE,  THERMOMETERS,  EXPANSION. 

Experiment  168.  — Rub  a  brass  button  on  the  floor  or  carpet 
it  soon  becomes  uncomfortably  warm. 

Experiment  169.— Place  a  nail  or  coin  on  an  anvil  or  stone  and 
hammer  it  vigorously ;  it  soon  becomes  too  hot  to  handle.  In 
this  way,  blacksmiths  sometimes  heat  iron  rods  to  redness. 

Experiment  170.— Hold  a  piece  of  iron  or  steel  against  a  dry 
grindstone  in  rapid  revolution ;  the  shower  of  sparks  noticeable  ia 
due  to  the  fact  that  the  small  particles  of  metal  torn  off  by  the 
grindstone  are  heated  to  incandescence. 

354.  Heat  Produced  by  Mechanical  Energy.— 
Tlie  arresting  of  mechanical  motion  transforms 
visible  energy  into  heat.    (See  §  99.) 

355.  A  Day  Dream. — As  our  young  philosopher  notices  tha 
quickly  falling  blows  of  the  blacksmith's  hammer  on  the  nail  and 
anvil,  he  begins  to  think  very  intently.    He  knows  that  the  descend/ 


282  NATURAL  PHILOSOPHY.  §  355 

ing  hammer  nas  energy  for  it  is  able  to  do  work.  He  understands 
that  this  is  so  because  the  hammer  has  weight  and  motion.  After 
the  last  blow  has  been  struck  and  the  hammer  lies  at  rest  on  the 
anvil,  he  begins  to  wonder  what  has  become  of  all  the  energy  that 
he  knows  was  in  the  hammer  when  it  was  in  motion  a  moment 
ago.  He  knows  that  the  visible,  kinetic  energy  has  disappeared, 
for  the  hammer  has  no  motion.  He  cannot  see  that  it  has  been 
transformed  into  potential  energy.  Yet  he  remembers  that  he  has 
been  told  that  energy,  like  matter,  is  indestructible.  Here  is  a  prob- 
lem ;  shall  he  give  it  up?  No,  for  he  is  our  "  young  philosopher." 

The  puzzling  problem  perplexes  him  so  that  be  falls  into  a  day- 
dream. His  "  scientific  imagination  "  begins  to  work.  He  again 
sees  the  falling  hammer;  he  hears  and,  with  quickened  percep- 
tion, almost  feels  the  shock.  The  motion  of  the  hammer,  as  a  ham- 
mer, ceases  ;  the  energy  of  the  hammer,  as  a  hammer,  is  destroyed. 
But  his  mind's  eye  sees  the  myriad  molecules  of  the  hammer,  nail 
and  anvil,  each  for  itself,  pick  up  a  portion  of  the  motion  lost  by 
the  hammer  as  though  the  shock  had  given  to  each  a  shiver. 
He  sees  that,  as  these  molecules  now  have  weight  and  motion, 
they  necessarily  have  kinetic  energy.  He  sees  that,  at  least, 
part  of  these  molecular  motions  were  produced  by  the  mass  motion 
of  the  hammer  and  that  the  total  quantity  of  energy  thus  gained 
oy  the  molecules  must  be  just  equal  to  the  quantity  of  energy  lost 
by  the  hammer. 

He  then  awakens  ;  he  sees  the  hammer,  nail  and  anvil  but  he 
cannot  see  the  molecular  motions  that  were  so  vivid  in  his  day- 
dream. He  places  his  hand  upon  the  iron  masses  and  finds  that 
they  were  heated  by  the  blows.  Like  Archimedes,  he  shouts 
"Eureka!  I  cannot  see  these  molecular  motions  but  I  can  feel 
them.  The  visible  energy  of  the  moving  mass  would  have  been 
jevealed  to  my  hand  as  a  crushing  blow  ;  the  invisible  energy  of 
the  moving  molecules  is  revealed  to  my  hand  as  heat." 

Experiment  171.— Pass  a  bent  glass  tube  through  the  air-tight 
cork  of  a  flask  half  full  of  water,  and  let  it  dip  beneath  the  sur- 
the  water.    Heat  the  flask.    The  heat  will  raise  some 


§358 


TEMPERATURE. 


283 


FIG.  174. 


of  the  water  to  the  end  of  the  tube  where  it  may  be  caught  as 
shown  in  Fig.  174. 

356.  Mechanical    En- 
ergy Produced  by  Heat. 
— Previous    experiments 
showed  us    that  mechanical 
energy    can    produce    heat. 
The  last  experiment  shows 
that  heat  can   perform   me- 
chanical   work:     it    lifted 
water  against   the  force  of 
gravity      There    certainly 
seems  to  be  a  close  con  nee 
tion  between  heat  and  the 
ordinary  forms  of  energy. 

357.  What  is  Heat? — Heat   is   a  form   of  en- 
ergy.   It  consists  of  the  ceaseless,  vibratory  motions 
of  the  molecules  of  matter  or  results  from  such 
motions  and  gives  rise  to  the  sensations  of  warmth 
and  cold. 

358.  What  is  Temperature  ? — The  temperature 
of  a  body  is  its  condition  considered  with  refer- 
ence to  its  ability  to  communicate  heat  to  other 
bodies. 

Temperature  is  a  very  different  thing  from  quantity 
jf  heat.  A  cup  of  water  dipped  from  a  lake  will  have 
the  same  temperature  as  the  lake,  but  the  water  in  the 
lake  will  have  incomparably  more  heat  than  the  water 
ua  the  cup. 


NATURAL    PHILOSOPHY. 


§359 


359.  Thermometers. — An  instrument  for  meas- 
uring temperature  is  called  a  thermometer.  The 
mercury  thermometer  is  the  most  common. 


360.  Thermometric  Scales. — There  are 
two  scales  used  in  this  country,  the  centigrade 
and  Fahrenheit's.  For  these  scales,  the  fixed 
points  are  marked  as  follows  : 

Centigrade.        Fahrenheit. 
Freezing  point,        0°  32° 

Boiling  point,       100°  212° 

The  tube  between  these  two  points  is  divided 
into  100  equal  parts  for  the  centigrade  scale 
and  into  180  for  Fahrenheit's.  Hence  a  change 
of  temperature  of  5°  C.  is  equal  to  a  change 
of  9°  F.,  or  an  interval  of  one  centigrade  degree 
is  equal  to  an  interval  of  -|  of  a  Fahrenheit  degree. 


Fiu.  175. 


Experiment  172. — Provide  a 
metal  ball  which,  at  ordinary 
temperatures,  will  easily  pass 
through  a  certain  ring ;  heat  the 
ball  and  it  will  no  longer  pass 
through  the  ring.  If  the  ball  be 
cooled  by  plunging  it  into  cold 
water,  it  will  again  pass  through 
the  ring.  (Fig.  176.) 

361  Expansion.  —  In- 
crease of  volume  is  the 
first  visible  effect  of  heat 
upon  bodies. 


FIG.  17ft, 


§  364  H&PAiratoX.  285 

Whatever  raises  the  temperature  of  a  body  increases 
:he  energy  with  which  the  molecules  of  that  body  swing 
to  and  fro.  Molecules  thus  vibrating  must  push  each 
other  further  apart,  and  thus  cause  the  body  which  they 
constitute  to  expand. 

Experiment  173.— Saw  a  piece  from  one  side  of  a  large  iron 
link  and  force  the  ends  of  the  opened  link  slightly  together  so 
that,  the  small  piece  may  be  pressed  hard  enough  to  hold  it  in 
place.  When  the  opposite  side  of  the  link  is  heated,  it  will  expand 
and  the  piece  will  fall  out  of  its  place. 

362.  Expansion    of  Solids. — The  energy  of  ex- 
pansion and  contraction  of  solids,  whan  heating  and  cool- 
ing, is  remarkable.     This  expansion  of  metals  by  heat 
is  utilized  by  coopers  in  setting  hoops,  by  wheelwrights 
in  setting  tires,  by  builders  in  straightening  bulging  walls, 
fe 

363.  Anomalous  Expansion  of  Water. — Water 
presents  a  remarkable  exception  to  the  general  rule.     If 
icatcr  at  0°  0.  be  heated,  it  will  contract  until  it 
readies  4°  C.,  its  temperature  of  greatest  density. 
Heated  tiborc  fit  is  point,  it  expands. 

364.  Results  of  this   Exception.— This  property 
of  water  is  of  great  importance.     Were  it  otherwise,  the 
ice   would    sink  and  destroy  everything  living  in  the 
water.     The  entire  body  of   water  would  soon   becoma 
a  solid  mass  which  the  heat  of  summer  could  not  wholly 
meh,  for,  ns  we  shall  soon  see.  wafer  has  little  power  to 
carry  heat  downward. 


286  KAtVRAL  PHILOSOPHY.  §  365 

Experiment  174.— Partly  fill  a  bladder  with  cold  air,  tie  up 
the  opening  and  place  the  bladder  near  the  fire.  The  air  will  ex- 
pand and  fill  the  bladder. 

Experiment  175. — Heat  a  closed  flask  having  a  delivery  tube 
terminating  under  water. 
Some  of  the  expanded  air 
will  be  forced  to  escape,  and 
may  be  seen  bubbling 
through  the  water.  By 
"  collecting  over  water  "  the 
air  thus  driven  out,  it  may 
be  accurately  measured. 
(Fig.  177.) 


Experiment  176.  —  Cut 
a  flat  spiral  with  two  or 
three  turns  from  a  piece  of 
stiff  writing  paper  about 
three  inches  in  diameter 
thrust  a  pin  through  the 
centre  of  the  paper,  take 
the  pin  by  the  point  and 
FIG.  177.  bold  the  spiral  over  a  hot 

stove  or  lighted  lamp.  As- 
cending currents  of  heated  air  will  cause  the  spiral  to  turn  alxmt 
the  pin  as  an  axis. 

365.  Expansion  of  Gases. — The  ascension  of  "fire 
balloons"  and  the  draft  of  chimneys  are  due  to  the  ex- 
pansion of  gases  by  heat.  When  the  air  in  the  chimney 
of  a  stove  or  lamp  is  heated,  it  is  rendered  lighter  than 
the  same  bulk  of  surrounding  air  and,  therefore,  rises. 
The  cooler  air  conies  in  to  take  its  place. 


§367 


R  EGA  PITULA  TION. 


287 


366.  Absolute  Zero  of  Temperature. — Tlie  tern 
perature  at  which  the  molecular  motions  constitut- 
ing heat  wholly  cease  is  called  the  absolute  zero. 
It  has  never  been  reached,  and  has  been  only  approxi. 
mately  determined. 

(a.)  Temperature,  when  reckoned  from  the  absolute  zero,  ia 
called  absolute  temperature.  Absolute  temperatures  are  obtained 
by  adding  460  to  the  reading  of  a  Fahrenheit  thermometer, 
or  273  to  the  reading  of  a  centigrade  thermometer. 

367.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


DEFINITION. 

PRODUCED  BY  FRICTION. 
INVISIBLE  MOLECULAR  ENERGY. 
DEFINITION. 


TEMPERATURE. 


EXPANSION.,.. 


THERMOMETERS 

ABSOLUTE. 
CAUSE. 
Souus. 
LIQUIDS. 


(•Centigrade.  1 

L 

I  Fahrenheit.  I 


NATURAL  PHILOSOPHY.  §  367 


EXERCISES. 

1.  If  a  centigrade  thermometer  records  15°,  what  will  be  th« 
reading  of  a  Fahrenheit  thermometer  by  its  side  ? 

2.  Express  a  temperature  of  59°  F.  in  degrees  centigrade. 

8.  What  is  the  absolute  temperature  of  15°  C.  ?    What  does 
your  answer  mean  ? 

4.  How  can  you  easily  show  that  mechanical  energy  may  be 
converted  into  heat  energy  ? 

5.  Why  does  not  Lake  Erie  freeze  solid  to  the  bottom  ? 

6.  Show  how  the  draft  of  a  chimney  depends  upon 


SECTION     II. 


LIQUEFACTION  AND   VAPORIZATION. 

Experiment  177. — Melt  some  ice  in  any  convenient  dish 
When  it  is  partly  melted,  stir  the 
liquid  part  with  a  chemical  thermometer 
and  notice  carefully  the  temperature  at 
which  ice  melts.  Frequently  deter- 
mine the  temperature  until  the  ice  is 
all  melted  and  notice  that  the  tem- 
perature is  constant  until  the  last 
solid  particle  disappears. 


pIG 


Experiment  178.  —  Repeat  the  ex- 
periment with  tallow,  beeswax  and 
Bulphur  in  succession.  It  may  be  ob- 
served, in  each  case,  that  the  hotter 
the  fire,  the  more  rapidly  the  solid  will 
melt  but  that  there  will  be  no  rise  of 
temperature  from  the  time  the  solid 
begins  to  melt  until  it  is  all  melted. 
Heat  the  sulphur  somewhat  above  the 

melting  poi  it  and  allow  it  to  cool.     Notice  the  temperature  when 
it  begins  to  solidify  and  while  it  is  solidifying 

368.  Laws  of  Fusion.  —  It  has  been  found  by  ex- 
periment that  the  following  statements  are  true  : 

(1.)  Evefy  solid  begins  to  melt  at  a  certain  tern* 

perature  wliicli,  for  a  given  substance,  is  invariable 

if  the  pressure  be  constant.     When  cooling,  the  sub- 

stance will  solidify  at  the  temperature  of  fusion. 

13 


290  NATURAL  PHILOSOPHY.  §368 

(2.)  TJie  temperature  of  the  solid  or  liquid  re- 
mains at  the  melting  point  from  the  moment 
that  fusion  or  solidification  begins  until  it  is 
complete. 

369.  Vaporization.— If,  after  liquefaction,  further 
additions  of  heat  be  made,  a  point  will  be  reached  at 
which  the  liquid  will  pass  into  the  aeriform  condition. 
This  change  of  form  is  called  vaporization.    Vaporization 
is  of  two  kinds — evaporation  and  ebullition. 

370.  Evaporation.  —  Evaporation   signifies    the 
quiet  formation  of  vapor  at  the  surface  of  a  liquid. 

371.  Ebullition. — Ebullition  or  boiling,  signifies 
the  rapid  formation  of  vapor  bubbles  in  the  mass 
of  a  liquid.     (See  Elements  of  Natural  Philosophy, 
§  501.) 

Experiment  179. — Tn  a  beaker  half  full  of  water,  place  a  ther- 
mometer and  a  test  tube  half  filled  with  ether.  (Fig.  179.)  Heat 
the  water.  When  the  thermometer  shows  a  temper- 
ature of  about  00°  C.,  the  ether  will  begin  to  boil. 
The  water  will  not  boil  until  the  temperature  rises 
to  100°  0.  The  temperature  will  not  rise  beyond 
this  point. 

Experiment  180.— Half  fill  a  Florence  flask  with 
water.     Boil  the  water  until  the  steam  drives  the 
Jj'ia.    179.       air  from  the  upper  part  of  the  flask.     Cork  tightly, 
remove  the  lamp  and  invert  the  flask.     (Fig.  180.) 
The  exclusion  of  the  air  may  be  made  more  certain  by  immers- 
ing the  corked  neck  of  the  tiask  in  water  that  has  been 
boiled. 


§372 


BOILING. 


291 


When  the  lamp  was  removed,  the  temperature  was  not  abov« 
100°  ( '.  By  the  time  that  the  flask  is  inverted  and  the  boiling  ha3 
ceased,  the  temperature  will  have  fallen  below  100°  C.  When  the 
boiling  stops,  pour  cold  water  upon  the  flask  ;  directly  the  boil 
ing  begins  again. 

The  cold  water  poured  upon  the  flask  lowers  the  temperature  oi 
the  water  in  the  flask  still  further,  but  it  also  condenses  some  or 
the  steam  in  the  flask  01  reduces  its  tension  (§  179).  This  reduc- 
tion of  the  tension  lessens  the  work  necessary  to  boiling.  There 
being  enough  heat  iu  the  water  to  do  this  lessened  amount  of 
work,  the  water  again  boiteand  increases  the  pressure  until  the 
boiling  point  is  raised  above 
the  temperature  of  the 
water. 

The  flask  may  be  drenched 
and  the  water  made  to  boil 
a  dozen  times  in  succession 
with  a  single  heating.  The 
experiment  may  be  made 
more  striking  by  plunging 
the  whole  flask  under  cool 
water. 

While  water  boils  at  212° 
P.  under  a  pressure  of  one 
atmosphere,  it  must  be 
heated  to  about  2oO°  F.  to 
boil  under  a  pressure  of 
two  atmospheres  or  to  more 
than  350  °  F.,  under  a  pres- 
sure of  ten  atmosjvhfn\s.  rio. 

372.  Laws  of  Ebullition. 

(1.)   Even/   li(/ni<J    ln'giiis   to   boil   at   a    certain 
temperature,  which,  for  it  given  substance,  is  in- 


292  NATURAL   PHILOSOPHY.  §  372 

variable  if  the  pressure  be  constant.    Wlien  cooling, . 
ttie  substance  will  liquefy  at  the  boiling  point. 

(2.)  TJie  temperature  of  the  liquid,,  or  vapor, 
remains,  at  the  boiling  point  from  the  moment 
that  it  begins  to  boil  or  liquefy. 

(3.)  An  increase  of  pressure  raises  the  boiling 
point;  a  decrease  of  pressure  lowers  the  boiling 
point. 

373.  Concerning  Steam. — A  given  mass  of  wa- 
ter in  the  aeriform  condition  occupies  nearly 
1700  times  as  much  space  under  a  pressure  of 
one  atmosphere  as  it  does  in  the  liquid  condi- 
tion. In  other  words,  a  cubic  inch  of  water  will  yield 
pearly  a  cubic  foot  of  steam. 

Steam  is  invisible.  What  is  commonly  called  steam 
is  not  true  steam,  but  little  globules  of  water  condensed 
by  the  cold  air  and  suspended  in  it.  By  carefully 
noticing  the  steam  issuing  from  the  spout  of  a  tea-kettle, 
it  will  be  observed  that  for  about  an  inch  from  the  spout 
there  is  nothing  visible.  The  steam  there  has  not  had 
opportunity  for  condensation.  The  water  particles  vis- 
ible beyond  this  space  passed  through  it  as  invisible 
steam.  The  steam  in  the  flask  of  Fig.  180  is  invisible. 

Experiment  181. — Boil  water  colored  with  ink  in  the  flask,  a. 
/Pig.  181.)  The  steam  passes  through  the  delivery  tube  into  c 
which  is  placed  in  a  vessel  of  iced  water.  The  steam  that  con- 
denses  in  the  delivery  tube  and  the  smaller  flask  will  be  found  to 
be  colorless  water.  The  ink  has  been  left  In  a. 


§374 


DISTILLATION. 


374.  Distillation.— Distil- 
lation is  a  process  of  sepa- 
rating a  liquid  from  a 
solid  which  it  holds  in 
solution,  or  of  separating  a 
mixture  of  two  liquids  hav- 
ing different  boiling  points. 
The  process  depends  upon  the 
fact  that  different  substances 
are  vaporized  at  different  tem- 
peratures. 

The  apparatus,  called  a 
still,  is  made  in  many  forms, 

but  consists  essentially  of  two  parts — the  retort  for  pro 
ducing  vaporization,  and  a  condenser  for  changing  the  v» 


FIG.  181. 


Fw.  183. 


294 


NATURAL  PHILOSOPHY. 


§374 


por  back  to  the  liquid  form.  Fig.  182  represents  one  form 
of  the  apparatus.  It  consists  of  a  retort,  db,  the  neck 
of  which  is  connected  with  a  spiral  tube,  dd,  called  the 
worm.  The  worm  is  placed  in  a  vessel  containing  water. 
This  vessel  is  continually  fed  with  cold  water  carried  to 
the  bottom  by  the  tube  7i.  As  the  water  is  warmed  by 
the  worm  it  rises  and  overflows  at  i. 

(a. )  Suppose  that  water  is  to  be  separated  from  the  salt  it  holds  in 
solution.  The  brine  is  placed  in  a  retort  and  heated  a  little  above 
212°  F.  At  this  temperature,  the  water  is  vaporized  while  the  salt 
is  not.  The  steam  is  driven  from  the  retort  through  the  worm, 
where  it  is  rapidly  condensed  and  passes  into  a  vessel,  g,  prepared 
to  receive  it.  The  salt  remains  in  the  retort,  a. 


FIG.  183. 

(&.)  Fig.  183  represents  a  simpler  form  of  apparatus  for  this 
purpose.  The  retort  is  a  Florence  flask,  Ft  the  delivery  tube  of 
which  passes  through  a  "  water-jacket,''  J.  The  method  of  supply 
tng  this  condenser  with  cold  water  is  evident  from  the  figure. 

(0.)  Suppose  that  alcohol  is  to  be  separated  from  water.     The 


§375 


RECAPITULATION. 


29.7 


solution  is  placed  in  the  retort  and  heated  about  90°  C.,  which  ig 
above  the  boiling  point  of  alcohol  but  below  that  of  water.  The 
alcohol  will  pass  over  in  a  state  of  vapor  and  be  condensed,  while 
the  water,  etc.,  remain  behind. 

375.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


CHANGE     OF 

PHYSICAL 
QQUDITION. 


LIQUEFACTION.— LAWS. 

EVAPORATION. 

VAPORIZATION. 

BOILING. 


Definition. 

Laws. 

Effect  of  Pressure 

Steam. 

Di*till*ti<m. 


NATURAL  PHILOSOPHY.  §  375 


EXERCISES. 

1.  Sulphur  begins  to  melt  at  115°  C.     At  which  temperature 
will  melted  sulphur  begin  to  solidify? 

2.  What  is  the  difference  between  evaporation  and  ebullition  1 

3.  How  can  water  be  heated  above  the  ordinary  boiling  point  t 

4.  On  the  top  of  high  mountains,  water  boils  at  so  low  a  tem- 
perature, owing  to  diminished  atmospheric  pressure,  that  it  does 
not  become  hot  enough  to  cook  many  kinds  of  food.    Can  you  sug 
geet  a  remedy  for  this  difficulty  ? 

5.  What  does  steam  look  like? 

6.  What  fact  underlies  all  processes  of  distillation  ? 

7.  How  may  sea  water  be  rendered  tit  lor  drinking? 

8.  Ice  and  snow  are  being  melted  in  a  kettle  over  a  hot  fire. 
How  warm  can  they  be  made  before  they  are  wholly  melted  ? 

9.  A  cubic  foot  of  water  is  vaporized,     (a.)  What  is  the  volume 
of  the  steam  under  a  pressure  of  one  atmosphere  ?    (6.)  Under  g 
pressure  of  two  atmospheres?    (See  Exercise  7,  page  137.) 


SECTION      I  I  I. 

LATENT   AND   SPECIFIC   HEAT. 

376.  Thermal   Units. — A  thermal  or  heat  unit 
is  the  amount  of  heat  necessary  to  warm  a  weight 
unit  of  water  one  degree  above  the  freezing  point. 
The   weight  unit  generally  used  is  the  gram  or  pound  ; 
any  other  weight  unit  may  be  used  with  equal  propriety. 
The  degree  may  be  centigrade  or  Fahrenheit. 

(a.)  We  have  three  units  in  common  use.     They  are  the  amounts 
of  heat  necessary  to  warm 

(1.)  A  kilogram  of  water  from  0°  C.  to  1°  C.     (A  calorie.) 
(2.)  A  gram  of  water  from  0°  C.  to  1°  C.     (A  lesser  calorie./ 
(3.)  A  pound  of  water  from  32°  F.  to  33 D  F. 

Experiment  182.— Take  a  block  of  ice  at  a  temperature  of 
-10  C.  (14°  F.)  and  warm  it.  A  thermometer  placed  in  it  rises  to 
0°  C.  The  ice  begins  to  melt,  but  the  mercury  no  longer  rises. 
Heat  is  still  applied  but  the  thermometer  remains  stationary 
until  the  last  particle  of  ice  has  been  liquefied.  Then  the  tem- 
oerature  begins  to  rise  and  continues  to  do  so  until  the  ther- 
mometer marks  100°  C.  The  water  then  begins  to  boil,  and 
the  temperature  a  second  time  becomes  fixed.  See  Element* 
of  Natural  Philosophy,  §§  580-582. 

377.  Definition    of    Latent    Heat.— The    latent 
heat  of  a  substance  is  the  quantity  of  heat  that 
must  be   communicated    to  a  body  to   change  its 
condition  without  changing  its  temperature. 


298  NATURAL   PHILOSOPHY.  §  377 

It  may  be  made  to  reappear  as  sensible  heat  during  the 
opposite  changes,  after  any  interval  of  time. 

378.  Latent  Heat  of  Fusion. — When  ice  or  any 
other  solid  is  melted  by  the  direct  application  of  heat, 
much  of  the  heat  is  rendered  latent.     We  may  represent 
the  process  of  liquefaction  of  ice  as  follows  : 

Water  at  0°  C.  =  ice  at  0°  C  +  latent  heat  of  water. 

Experiment  183. — Mix  two  weights  of  pulverized  ammonium 
nitrate  and  one  weight  of  pulverized  ammonium  chloride  (sal  am- 
moniac) and  dissolve  the  mixture  in  three  weights  of  cold  water, 
stirring  the  substances  together  with  a  small  test  tube  containing 
a  little  cold  water.  Determine  the  temperature  of  the  dissolved 
mixture  with  a  chemical  thermometer  and  notice  the  condition 
of  the  water  in  the  test  tube. 

379.  Latent  Heat  of  Solution. — During  the  pro- 
cess of  solution,  as  well  as  during  fusion,  heat  is  ren- 
dered latent.     Hence,  the   solution   of  a  solid   in- 
volves a  diminution  of  temperature. 

(a.)  A  cup  of  coffee  is  cooled  by  sweetening  it  with  sugar  and  a 
plate  of  soup  is  cooled  by  flavoring  it  with  salt. 

380.  Freezing  Mixtures. — The  latent  heat  of  so- 
lution lies  at  the  foundation   of   the   action   of 
freezing  mixtures.     For  example,  when  ice  is  melted 
by  salt  and  the  water  thus  formed  dissolves  the  salt  itself, 
the  double  liquefaction  requires  a  deal  of  heat  which  is 
often  furnished  by  the  cream  in  the  freezer. 

(a.)  The  freezing  mixture  roost  commonly  used  consists  of  one 


§  38 1  LATENT  HEAT.  299 

weight  of  salt  and  two  weights  of  snow  or  pounded  ice.  The, 
mixture  assumes  a  t§mperature  of  — 18°  C.,  which  furnished  the 
zero  adopted  by  Fahrenheit. 

(6.)  Persons  who  sleep  in  cold  chambers  sometimes  notice  that 
as  soon  as  they  touch  a  pitcher  of  water  that  has  been  standing  in 
the  room  over  night,  the  water  quickly  freezes.  If  a  particle  of 
ice  be  dropped  into  the  water,  the  same  result  follows.  We  may 
say  that,  in  this  condition,  liquids  have  a  tendency  to  become 
solid  and  are  restrained  only  by  the  difficulty  of  making  a  be- 
gLnniag. 

Experiment  184. — Surround  with  a  freezing  mixture,  a  small 
glass  vessel  containing  water  and  a  mercury  thermometer.  The 
temperature  of  the  water  may  be  reduced  to  —10°  C.  or  —12°  C. 
without  freezing  the  water.  A  slight  movement  of  the  thermome- 
ter in  the  water  starts  the  freezing  and  the  temperature  quickly 
rises  to  0°  C. 

Experiment  185.— Dissolve  two  weights  of  Glauber's  salt  in  one 
weight  of  hot  water,  cover  the  solution  with  a  thin  layer  of  oil 
and  al!ow  to  cool,  in  perfect  quiet,  to  the  temperature  of  the  room. 
By  plunging  a  thermometer  into  the  still  liquid  substance,  solidi- 
fication (crystallization)  is  started  and  the  temperature  rapidly 
rises.  Dr.  Arnott  found  that  this  experiment  was  successful  after 
keeping  the  solution  in  the  liquid  condition  for  five  years. 

Experiment  186. — To  three  weights  of  quicklime,  add  one  weight 
of  water.  The  water  will  be  completely  solidified  in  the 
slaking  of  the  lime  with  remarkable  manifestations  of  heat. 
Carts  containing  quicklime  have  been  set  on  fire  by  exposure  to 
heavy  rains 

381.  Solidification. — Solidification  signifies  the  pas- 
sage from  the  liquid  to  the  solid  condition.  During 
solidification,  tJtere  is  an  increase  of  temperature. 


300  NATURAL  PHILOSOPHY.  §  381 

The  heat  energy,  being  no  longer  employed  in  doing 
the  work  of  maintaining  liquidity,  is. reconverted  into 
sensible  heat. 


Experiment  187.— Pour  a  teaspoonful  of  sulphuric  ether  into 
the  palm  of  the  hand,  being  sure  that  there  is  no  flame  near  to 
ignite  the  inflammable  vapor  of  the  ether.  As  the  liquid  evapo- 
rates, notice  that  the  hand  is  furnishing  the  heat  needed  to 
perform  the  work  of  vaporization. 

Experiment  188. — Wet  a  block  of  wood  and  placea  watch  crystal 
upon  it.  A  film  of  water  may  be  seen  under  the  central  part  of  the 
glass.  Half  fill  the  crystal  with  sulphuric  ether  and  rapidly 
evaporate  it  by  blowing  over  its  surface  a  stream  of  air  from  a 
small  bellows.  So  much  heat  is  rendered  latent  in  the  vaporiza- 
tion that  the  watch  crystal  is  firmly  frozen  to  the  wooden 
block. 

Experiment    189. — 

Nearly  fill  the  porous 
cup  of  a  Grove  or  Bun- 
sen  cell  (§§  262,  263) 
with  water  at  the  tem- 
perature of  the  room. 
As  the  water  evapo- 
rates at  the  surface  of 
the  porous  cell,  the 
water  in  the  cell  be- 
comes cooler,  heat 
having  been  with- 
drawn for  the  work  of 
vaporization.  After 
the  lapse  of  15  or  20 
FIG.  184.  minutes,  use  a  chemi- 


§  383  LATENT  HEAT.  301 

cal  thermometer  to  determine  the  temperature  of  the  water.     In 
this  way,  water  is  often  cooled  in  tropical  regions. 

Experiment  190.— In  a  vessel  of  sulphuric  ether,  place  a  test 
tube  containing  water.  Force  a  current  of  air  through  the  ether. 
(Fig.  184.)  Rapid  evaporation  is  thus  produced  and,  in  a  few 
minutes,  the  water  is  frozen. 


382.  Latent  Heat  of  Vaporization.— The  vapor- 
ization of  a  liquid  is  accompanied  by  the  disappearance 
of  a  large  quantity  of  heat  and,  frequently,  by  a  dimi- 
nution of  temperature.     There  is  a  change  of  sensible 
into  latent  heat;  of  kinetic  into  potential  energy.    We 
may  represent  the  vaporization  of  water  as  follows: 

Steam  at  100°  C.  =  water  at  100°  C.  +  latent 
heat  of  steam. 

383.  Condensation  of  Gases. — Gases  may  be  con- 
densed by  union  with  some  liquid  or  solid,  by  cold  or  by 
pressure.     It  has  been  recently  shown  that  any  known 
gas  may  be  liquefied  by  cold  and  pressure.     In  any  case, 
the  condensation  of  a  gas  renders  sensible  a  large 
amount  of  heat. 

(a.)  When  a  gas  that  has  been  condensed  is  allowed  to  expand 
under  such  circumstances  that  ft  must  do  work  (e.g.  forcing  bach 
the  surrounding  air),  its  temperature  is  lowered.  Icicles  have 
l>een  formed  by  allowing  condensed  air  charged  with  watery  vapor 
to  escape  suddenly  from  the  confining  vessel.  The  rapid  expan- 
sion of  carbon  dioxide  ("  carl  onic  acid  gas")  when  a  bottle  of  beet 
or  champagne  is  opened,  often  produces  a  fog  in  the  neck  of  the 
bottle.  (See  C/temintry,  %  197.) 


302  NATURAL  PHILOSOPHY.  §  384 

(6.)  Carbon  dioxide  may  be  liquefied  by  great  pressure.  When 
this  liquid  escapes  through  a  small  orifice  into  the  air,  evapora- 
tion is  so  rapid  that  some  of  the  dioxide  is  frozen  in  the  form  of  a 
fine  snow.  This  carbon  dioxide  snow  dissolves  in  sulphuric  ether 
and  with  it  forms  one  of  the  most  intense  freezing  mixtures  kmown. 
By  aiding  the  evaporation  of  this  mixture  with  an  air  pump, 
Faraday  obtained  a  temperature  of  —110"  C. 

Experiment  191.— Mix  a  pound  of  ice-cold  water  (0°  C.)  with 
a  pound  of  water  at  80°  C.  Note  the  temperature  of  the  mixture. 
We  have  two  pounds  of  water  at  40°  C.  The  heat  lost  by  the 
hot  water  in  cooling  40  degrees  was  equal  to  the  heat  gained  by 
an  equal  weight  of  water  in  warming  40  degrees — in  fact  it  was 
identical  with  it. 

Experiment  192.— Place  a  pound  of  melting  ice  (0°  C.)  in  a 
pound  of  water  at  80°  C.  When  the  ice  is  all  melted,  note  the 
temperature  of  the  water.  We  have  two  pounds  of  water  at 
0°  C. 


384.  The  Latent  Heat  of  Water —In  Experi- 
ments 192  and  193,  the  heat  which  warmed  one  pound 
of  water  from  0°  to  80°  C.  was  used  to  melt  a  like 
weight  of  ice.  Hence,  by  definition,  the  latent  heat 
of  water  is  80°  C.  (or  144°  F.). 

TJie  amount  of  heat  required  to  melt  a  quan- 
tity of  ice  without  changing  its  temperature  is 
eighty  times  as  great  as  the  heat  required  to 
warm  the  same  quantity  of  water  one  centigrade 
degree. 

(a.)  Because  of  this  great  latent  heat  of  water,  the  processes  of 
melting  ice  and  freezing  water  are  necessarily  slow. 


§  386  LATENT  HEAT.  303 

385.  The  Latent  Heat  of  Steam.— The  amount 
of  heat  necessary  to  evaporate  one  gram  of  water  would 
suffice  to  raise  the  temperature  of  537  grams  of  water 
1°  C.     Hence,  the  latent  heat  of  steam  is  537°  C.  (or 
967°  F.). 

TJie  amount  of  heat  required  to  evaporate  a 
quantity  of  water  without  changing  its  tempera- 
ture is  537  times  as  great  as  the  heat  required 
to  warm  the  same  quantity  of  water  one  centi- 
grade degree. 

(a.)  When  a  gram  of  steam  is  condensed,  537  heat  units  (centi- 
grade-gram-water) are  liberated.  In  this,  we  see  an  explanation 
of  the  familiar  fact  that  scalding  by  steam  is  so  painfully  severe. 

(6.)  Were  it  not  for  the  latent  heat  of  steam,  when  water 
reached  its  boiling  point  it  would  instantly  flash  into  steam  with 
tremendous  explosion. 

386.  Problems  and  Solutions. — (1.)  How  many  grams  of  ice  at 
0°  C.  can  be  melted  by  one  gram  of  steam  at  100°  C.  ?    One  gram 
of  steam  at  100°  C.,  in  condensing  to  water  at  the  same  tempera- 
ture, parts  with  all  its  latent  beat,  or  537  heat  units.    The  gram  of 
water  thus  formed  can  give  out  100  more  heat  units.     Hence,  the 
whole  number  of  beat  units  given  out  by  the  steam  in  changing 
to  water  at  0°  C.,  the  temperature  at  which  it  could  no  longer 
melt  ice,  is  537  +  100=637. 

Since  it  requires  80  heat  units  to  melt  one  gram  of  ice,  637  heat 
units  will  melt  as  many  grams  as  80  is  contained  times  in 
637,  which  is  7.96.  Therefore,  the  steam  will  melt  7.96  grams  of 
ice. 

(2.)  How  many  pounds  of  steam  at  100°  C.  will  just  melt  100 
pounds  of  ice  at  0°  C.  ?  To  melt  100  pounds  of  ice,  (80  x  100=), 
8,000  heat  units  will  be  required.  Each  i>ound  of  steam  can  fur- 


304  NATURAL   PHILOSOPHY.  §  386 

nish  63?  heat  units  for  the  work  required.     8,000  -H  637  =  12.55. 
the  number  of  pounds  of  steam. 

(3.)  What  weight  of  steam  at  100°  C.  would  be  required  to 
raise  500  grams  of  water  from  0D  C.  to  103  C.  ?  Each  gram  o* 
water  will  require  10  heat  units  ;  500  grams  of  water  will  require 
5000  heat  units.  Each  gram  of  steam  can  furnish  (537  +  90  —)  627 
heat  units  for  the  work  required.  5000  -*-  627  =  7.97,  the  number 
of  grams  of  steam. 

Experiment  193. — Mix  any  convenient  quantity  of  ice-cold 
water  (0°  C.)  with  the  same  quantity  of  water  at  a  temperature 
of  30°  C.  Ascertain  the  temperature  of  the  mixture;  jt  will  be 
about  15°  C.  If  you  used  a  kilogram  of  water  in  each  case,  the 
15000  heat  units  lost  by  the  warm  water  and  the  15000  heat  units 
gained  by  the  cold  water  were  identical. 

387.  Equality  of  Loss  and  Gain. — A  substance 
loses  as  much  heat  in  cooling  a  given  number 
of  degrees  as  would  be  gained  by  a  like  quantity 
when  warmed  an  equal  number  of  degrees. 

Experiment  194. — Suspend  a  vessel  containing  3  kilograms  of 
mercury  in  boiling  water  until  you  are  sure  that  it  has  the  tem- 
perature of  the  water  (100°  C.)  Quickly  pour  the  mercury  into  one 
kilogram  (or  liter)  of  ice  cold  water,  0°  C.  The  temperature  of 
the  mixture  will  be  about  9°  C. 

The  kilogram  of  water,  in  being  warmed  9°  C.,  gained  9000 
heat  units ;  consequently,  the  3  kilograms  of  mercury  in  cooling 
91°  C.,  must  have  lost  9000  heat  units,  for  our  experiment  has 
neither  created  nor  destroyed  any  heat  and,  if  it  was  neatly  per- 
formed, but  little  was  lost. 

For  each  degree  of  change  of  temperature,  the  kilogram  of  water 
gained  1000  heat  units  ;  for  each  degree  of  change  of  temperature 

9000 

one  kilogram  of  mercury  lost  ^  -=  =  33  heat  units.     In  other 
yi  x  o 

words,  it  takes  about  30  times  as  much  heat  to  warm  a  giveq 


§  3«9  SPECIFIC  HEAT.  305 

yeight  of  water  one  degree  as  it  does  thus  to  warm  the  same 
weight  of  mercury. 

388.  Definition  of  Specific  Heat.— The  specific 
heat  of  a  body  is  the  ratio  between  the  quantity 
of  heat  required   to  warm  it  one  degree  and  the 
quantity  of  heat  required  to  warm  an  equal  weight 
of  water  one  degree. 

(«.)  It  is  very  important  to  bear  in  mind  that  specific  heat,  like 
specific  gravity,  is  a  ratio  ;  nothing  more  nor  less.  The  8|>ecific 
heat  of  water,  the  standard,  is  unity.  The  specific  heat  of  iron  is 
0.1138;  of  copper,  0.0902;  of  tin,  0.0563  ;  of  lead,  0.0314  ;  of  bis- 
muth, 0.0308  ;  of  ice,  0.5  and  of  steam,  0.48.  These  ratios  will  be 
the  same  for  any  given  substance,  whatever  the  thermal  unit  or 
ihermometric  scale  adopted. 

389.  Heated  Balls  Melting  Wax.— The  difference 
between  bodies  in  respect  to  specific  heat  may  be  roughly 
illustrated  as  follows  :  small  balls  of  equal  weight,  mad« 
severally  of  iron,  copper,  tin,  lead  and  bismuth  are  heated 
to  a  temperature  of  180°  or  200°  C.  by  immersing  them 
in  hot  oil  until  they  all  acquire  the  temperature  of  the 
qil.     They  are  then  placed  on  a  cake  of  beeswax  about 


306 


NATURAL   PHILOSOPHY. 


§389 


half  an  inch  thick.  The  iron  and  copper  will  melt  their 
way  through  the  wax,  the  tin  will  nearly  do  so,  while  the 
lead  and  bismuth  sink  not  more  than  half  way  through 
the  wax.  This  shows  that  the  iron  or  copper  had  more 
heat  than  an  equal  weight  of  lead  or  bismuth  at  the  same 
temperature. 

390.  Specific   Heat   of   Water. —  Water   in  its 
liquid  form,  has  a  higher  specific  heat  than  any 
other  substance  except  hydrogen.    For  this  reason, 
the  ocean  and  lakes  are  cooled  and  heated  more  slowly 
than  the  land  and  atmosphere.     They  thus  modify  sud- 
den changes  of  temperature,  and  give  rise  to  land  and 
sea  breezes  and  to  the  well  known  fact  that  the  climate  of 
the  sea-coast  is  warmer  in  winter  and  cooler  in  summer 
than  that  of  inland  places  of  the  same  latitude. 

391.  Recapitulation.— To  be  amplified  by  the  pupil 
for  review. 

UNITS. 


HEAT. 


ENERGY.. 


KINETIC=SENSIBLE  HEAT. 


Convertibility  on 
'  Critical  occasions.1 


I.  POTENTIAL = LATENT  HEAT   • 


Definition. 

Of  Fusion. 

Of  Solution. 

Of  Water. 

Of  Steam. 

Freezing  Mixture* 

Solidification. 


DEFINITION. 

SPECIFIC.  I  ILLUSTRATION. 
I  OF  WATER. 


8  39*  EXERCISES.  307 


EXERCISES. 

1.  Why  does  sprinkling  the  floor  of  a  room  on  a  warm  day  add 
to  the  con:  fort  of  the  occupants  of  the  room  ? 

2.  Does  a  dog's  lolling  his  tongue  on  a  summer  day  render  him 
less  warm  ?    Explain. 

3.  Why  do  we  often  bathe  a  fevered  brow  with  water  or  with  a 
mixture  of  alcohol  and  water  ? 

4.  How  many  heat  units  are  required  to  melt  three  pounds  of 
ice  ?  Am.  432  units  (pound-Fahrenheit). 

5.  How  much  heat  is  required  to  raise  three  pounds  of  ice-cold 
water  to  the  boiling  temperature  ?  Ann.  540  units. 

6.  How  much  heat  is  required  to  vaporize  3  grams  of  boiling 
water?  Aw.  1611  lesser  calories. 

7.  How  much  heat  is  needed  to  melt  and  evaporate  8  pounds 
of  ice  ?  AM.  3873  units  (pound-Fahrenheit). 

8.  How  much  heat  is  needed  to  change  a  kilogram  of  ice  at 
-10°  C.  to  water  at  15°  C.  ?    (Take  note  of  the  specific  heat  of  ice.) 

Ans.  100  calories,  or  100,000  lesser  calories. 


SECTION     IV. 

MODES  OF  DIFFUSING   HEAT. 

Experiment  195.— Place  one  end  of  an  iron  poker  into  the  firfe 
The  other  end  soon  becomes  too  warm  to  handle. 

392.  Conduction.— TJie  process  by  which  heat  is 
transferred  from,  the  hotter  to  the  cold  v  parts  of 
a  body,  passing  from  one  molecule  to  tlie  next 
nioteviUe,  is  called  conduction  of  heat.  The  propa- 
gation is  very  gradual  and  as  rapid  through  a  crooked 
as  through  a  straight  bar. 

Experiment  196. — Instead  of  the  iron  poker  of  Experiment  195, 
use  a  glass  rod  or  wooden  stick.  The  end  of  the  rod  may  be 
melted  or  the  end  of  the  stick  burned  without  rendering  the 
other  end  uncomfortably  warm. 

Experiment  197. —Thrust  a  silver  and  a  German-silver  spoon 
into  the  same  vessel  of  hot  water;  the  handle  of  the  former  will 
become  much  hotter  than  that  of  the  latter. 

Experiment  198. — Place  a  bar  of  iron  and  one  of  copper  end  to 
end  so  as  to  be  heated  equally  by  the  flame  of  the  lamp.  Fasten 
small  balls  (or  nails)  by  wax  to  the  under  surfaces  of  the  bars  ?t 


§  393  MODES   OF  DIFFUSING   HEAT.  309 

equal  distances  apart.    More  balls  may  be  melted  from  the  cop- 
per than  from  the  iron. 

393.  Difference  in  Conductivity.— These  experi- 
ments show  that  some  substances  are  good  conductors 
of  heat  and  that  some  are  not. 

(a.)  Relative  thermal  conductivity  of  some  metals : 

Silver 100  Lead 9 

Copper 74  Platinum 8 

Brass 24  GernuJi  silver 6 

Iron 12  Bismuth 2 

The  above-named  metals  arrange  themselves  in  the  same  order 
with  reference  to  the  conduction  of  electricity,  silver  being  the 
best  and  bismuth  the  poorest.  This  relation  suggests  a  similarity 
of  nature  between  these  two  forms  of  energy. 

Experiment  199. — Pass  the  tube  of  an  air  thermometer  or  of 
an  inverted  mercury  thermometer  through  a  cork  in  the  neck  of  a 
funnel.     Cover  the    thermometer    bulb    to 
the    depth   of    about    half   an    inch    with 
water.     Upon  the  water,  pour  a  little  sul- 
phuric ether   and   ignite   it.     The    heat  of 
the  flame   will  be   intense   enough   to  boil 
a  small  quantity  of  water  held  over  it,  but 
the  thermometer  below  will   be  scarcely 
affected. 

Experiment   200.— Fasten    a   piece   of 
ice  at  the  bottom  of  a  glass  test  or  igni- 
tion  tube   and    cover  it  to   the   depth   of 
several  inches  with  water.     Hold  the  tube 
obliquely  and  apply  the  flame  of  a  lamp  be- 
low the  upper  part  of  the  water.    The  water  there  may  be  made 
to  boil  without  melting  the  ice  beiow.     Instead  of  using  ice  and 
water,  pack  the  tube  full  of  moist  snow  if  you  can  get  it. 


310  NATURAL  PHILOSOPHY.  §  394 

394.  Conductivity  of  Fluids. — Liquids  and  aeri- 
form bodies  are  poor  conductors  of  heat.  The  sui> 
face  of  a  liquid  may  be  intensely  heated  without  sensibly 
affecting  the  temperature  an  inch  below.  The  conduc- 
tivity of  iron  is  about  80  times  that  of  water ;  that  of 
copper  is  about  500  times  that  of  water,  'f  he  conduc- 
tivity of  gases  is  probably  less  than  that  of  liquids. 

Experiment  201.— Drop  a  small  quantity  of  cochineal  or  oak 
sawdust  into  a  glass  vessel  containing 
water.  Heat  the  water  by  a  lamp 
placed  below.  Notice  the  currents 
indicated  by  the  motion  of  the  solid 
particles. 

395.  Convection.  —Fluids 
(with  the  exception  of  mercury 
which  is  a  metal)  being  poor 
conductors,  they  cannot  be 
heated  as  solids  generally  are. 
Water,  e.g.,  must  be  heated 
from  beloiv ;  the  heated  mole- 
cules expand  and  rise  while  the 
FIG  188  cooler  ones  descend  to  take  their 

place  at  the  source  of  heat.    TJiis 

method   of  diffusing  heat,   by  actual  motion  of 
heated  fluid  masses,  is  called  convection. 

396.  Luminiferous  Ether. — In  the  case  ot  kinetic, 
mechanical  energy,  the  rapid  motion  of  bodies,  e.  g.,  a 
vibrating  guitar  string,  is  partly  carried  off  by  the  air  in 
lb.£  shnpe  ut  SQUUCJ., 


§  397  MODES  OF  DIFFUSING  HEAT.  311 

There  is  sufficient  reason  for  believing  that  thert 
is  a  medium  pervading  all  space  which  carrier 
off  part  of  the  invisible  motions  of  molecules,  just 
as  the  air  carries  off  a  portion  of  the  motion  of 
moving  masses.  This  medium  is  called  the  -lu~ 
jniniferous  ether. 

It  is  supposed  to  occupy  intermolecular  as  well  as  inter- 
planetary space,  and  to  pass  as  freely  between  the  particles 
of  ordinary  matter  as  the  winds  do  between  the  trees  of 
the  forest. 

Experiment  202.— Take  a  white-hot  poker  into  a  dark  room. 
It  emits  heat  and  a  white  light.  The  light  gradually  becomes 
reddish  and  less  bright,  and  finally  fades  from  view  as  a  dull  red 
glow.  Long  after  it  has  ceased  to  be  visible,  the  poker  continues 
to  give  heat  to  surrounding  objects,  as  may  be  shown  by  holding 
the  hand  or  face  near  it  on  any  side,  above  or  below.  There  has 
been  a  continuous  change  from  the  emission  of  white  light  and 
much  heat  to  that  of  no  light  and  less  heat. 

397.  Radiant  Heat.— The  molecules  of  a  heated 
body  are  in  a  state  of  active  vibration.  The  motion  of 
these  vibrating  molecules  is  communicated  to  the  ether 
and  transmitted  by  it,  as  waves,  with  wonderful  velocity. 
Thus,  when  you  hold  your  hand  before  a  fire,  the 
warmth  that  you  feel  is  due  to  the  striking  of  these 
ether-waves  upon  your  skin;  they  throw  the  nerves  into 
motion  just  as  sound-waves  excite  the  auditory  nerve 
and  the  consciousness  corresponding  to  this  motion  ia 
what  we  call  warmth. 

Heat  thus  propagated  by  the  ether,  instead  of  by 
ordinary  forms  of  matter,  is  called  radiant  heat. 


312  NATURAL  PHILOSOPHY.  §  397 

(a.)  From  the  last  experiment,  we  naturally  conclude  that 
radiant  heat  and  light  are  identical.  Thorough  experimental  in- 
vestigation has  confirmed  this  conclusion.  When  the  energy  of 
the  ether  waves  produces  a  cer!ain  effect  upon  the  retina  of  the  eye, 
we  call  it  light ;  when  it  produces  another  effect  upon  the  nerves 
of  touch,  we  call  it  radiant  heat.  "  Thus  radiant  heat  is  brought 
under  the  undulatory  theory  of  light,  which  in  its  turn  becomes  an- 
nexed as  a  magnificent  outlying  province  of  the  kinetic  theory  of 
teat."  (§  420.) 

398.  Radiation  of  Heat.  —  The  transfej-rence  of 
heat  from  one  body  to   another  at  a   distance  ir- 
respective of  the  temperature  of  the  intervening 
medium,  is  called  the  radiation  of  heat. 

399.  Incident  Rays. — When  radiant  heat  falls  upon 
a  surface,  it  may  be  transmitted,  reflected  or  absorbed. 
If  transmitted,  it  may  be  refracted.     Eock-salt  crystal 
transmits  nearly  all,   reflects  yery  little,   and  absorbs 
nardly  any.     Polished  silver  reflects  nearly  all,  absorbs 
a  little,  and  transmits  none.     Lampblack  absorbs  nearly 
all,  reflects  very  little,  and  transmits  none. 

Experiment  203.— Hold  a  pane  of  glass  between  the  face  and  a 
hot  stove.  Notice  that  the  glass  shields  the  face  from  the 
heat  of  the  stove.  Next,  hold  the  glass  between  the  face  and  the 
sun.  Notice  that  the  glass  does  not  shield  the  face  from  the 
heat  of  the  sun. 

400.  Diathermancy.— -Bodies  that  transmit  ra- 
diant   heat   freely    are     called    diathermanous ; 
those  that  do  not,  are  called  athermanous.     These 
terms  are  to  heat,  what  transparent  and  opaque  are 
to  light.     Bock  salt  Is  the  most  diathermous  substance 
known. 


§  402  MODES   OF  KTFFUSING   HEAT.  313 

401.  Obscure  and    Luminous   Heat. — Heat  that 
is  radiated  from  a  non-luminous  source,  as  from  a  ball 
heated  below  redness,  is  called  obscure  heat ;  while  part 
of  that  radiated  from  a  luminous  source,  as  from  the  sun 
or  from  a  ball  heated  to  incandescence,  is  called  lumi* 
nous  heat.     Heat  from  a  luminous  source  is  generally 
composed  of  both  luminous  and  obscure  rays. 

Glass,  water  or  a  solution  of  alum  allows  luminous  heat 
rays  to  pass  but  absorbs  nearly  all  of  the  heat  rays  from 
a  vessel  filled  with  boiling  water.  In  other  words,  these 
substances  are  diathermanous  for  luminous  rays  but 
athermanous  for  obscure  rays. 

A  solution  of  iodine  in  carbon  di-sulphide  transmits 
obscure  rays  but  absorbs  luminous  rays.  By  means  of 
these  substances,  luminous  and  obscure  rays  may  be 
sifted  or  separated  from  each  other. 

Experiment  204.— Hold  a  spectacle-glass  or  a  larger  lens 
(g  450)  from  an  opera  glass  in  the  sunlight,  perpendicular  to  the 
sun's  rays.  A  point  may  easily  be  found  below  the  lens  where  it 
is  unusually  warm.  At  this  point,  called  the  focus,  hold  the  tip 
of  a  common  friction  match  or  a  hit  of  gun  cotton  that  has  been 
blackened  with  lamp  black  or  soot.  The  concentrated  rays  of 
the  sun  will  set  fire  to  the  easily  combustible  substance.  GTIH 
cotton  may  be  ignited  at  the  focus  of  a  lens  made  of  ice. 

Experiment  205. — Stand  beside  an  open  fire  and  let  your  as- 
sistant stand  in  front  of  the  fire.  Let  him  use  a  piece  of  bright 
tin  with  which  to  throw  back  the  light  of 'the  fire  into  your  face. 
Notice  that  heat  as  well  as  light  is  thrown  back,  your  face  feel 
ing  warmer. 

402.  Refraction  and  Reflection  of  Heat. — Heat 
rays  may  be  bent  from  a  straight  line  on  entering  and 


314  NATURAL  PHILOSOPHY.  §402 

leaving  a  body,  as  shown  in  Experiment  204.  This 
bending  of  the  ray  is  called  refraction. 

The  refraction  of  obscure  rays  cannot  be  shown  by  a 
glass  lens,  since  glass  is  athermanous  for  such  rays.  A 
rock-salt  lens  and  a  thermopile  may  be  used  for  such  an 
experiment.  (See  §  278.) 

Kadiant  heat  may  be  reflected  like  light.     (See  §  430.) 

403.  Change  of  Radiant  into  Sensible  Heat— 
Of  all  the  rays  falling  upon  any  substance,  only  those 
that  are  absorbed  are  of  effect  in  heating  the  body  upon 
which  they  fall.    The  motion  of  the  ether  waves  may  be 
changed  into  vibrations  of ,   Dlecules  of  ordinary  matter, 
and  thus  produce  sensible  heat  but  the  same  energy  can- 
not exist  in  waves  of  ether  and  in  ordinary  molecular 
vibrations  at  the  same  time.     Lamp-black  is  a  remark- 
ably good  absorbent. 

Experiment  206. — Provide  two  small  tin  cans  of  the  same  size 
and  shape.  You  can  get  them  for  the  asking.  In  the  cover  oi 
each,  make  a  hole  through  which  you  may  pass  the  bulb  of  a 
chemical  thermometer.  Blacken  the  outside  of  one  can  with 
paint  or  candle  soot.  Fill  both  cans  with  hot  water  from  the  same 
vessel  and,  consequently,  of  the  same  temperature.  At  the  end  oi 
half  an  hour,  pass  the  bulb  of  the  thermometer  through  the  holes 
in  the  covers  and  ascertain  the  temperature  of  the  water  in  each 
jan.  It  will  be  found  that  the  blackened  can  has  radiated  its 
heat  (or  cooled)  more  rapidly  than  the  other. 

404.  Relations   of  Absorption,  Reflection  and 
Radiation.-Cr00$  absorbents  are  good  radiators  and 
poor  reflectors,  and  vice  versa.     The  powers  of  ab- 
sorption and  radiation  go  hand  in  hand.    (§  338.) 


§  405  RECAPITULATION.  315 

The  radiating  power  of  a  body  depends  largely  upon 
the  nature  of  its  surface ;  smoothing  and  polishing  the 
surface  increases  reflecting  power  and  diminishes  absorb- 
ing and  radiating  powers;  roughening  and  tarnishing 
the  surf^e  increase  the  absorbing  and  radiating  powers 
and  diminish  the  reflecting  power. 

405.  Recapitulation.— To  be  amplified  by  the  pupil 
for  review. 

DIFFUSION    OF   HEAT. 


316  NATURAL  PHILOSOPHY.  §  405 


EXERCISES. 

1.  Why  is  a  moist,  cold  atmosphere  more  severe  than  a  dry 
atmosphere  of  the  same  temperature  ? 

2.  Why  does  fanning  one's  self  on  a  warm  day  increase  one's 
personal  comfort? 

3.  Why  is  it  that,  in  a  cold  room,  some  things  seem  colder  than 
others  ? 

4  (a.)  Is  a  good  conductor  or  a  non-conductor  better  for  keeping 
a  warm  body  warm  ?    (6.)  For  keeping  a  cold  body  cool  ? 

5.  How  is  heat  transmitted  through  a  crooked  iron  rod  ? 

6.  Is  a  good  absorbent  of  heat  better  for  radiation  or  for  reflec 
tion  of  heat  ? 


SECTION     V. 

THERMODYNAMICS. 

406.  Definition  of  Thermodynamics.— Thermo- 
dynamics is  the  branch  of  science  that  considers 
the    connection    between    heat    and    mechanical 
work.     It  has  especial  reference  to  the  numerical  rela- 
tion between  the  quantity  of  heat  used  and  the  quantity 
of  work  done. 

407.  Correlation  of  Heat  and  Mechanical  En- 
ergy.— We  know  that  heat  is  not  a  form  of  matter  be- 
cause it  can  be  •  created  in  any  desired  quantity.    We 
must  continually  remember  that  it  is  a  form  of  energy. 
When  heat  is  produced,  some  other  kind  of  energy  must 
be    transformed.      Conversely,    when   heat  disappears, 
some  other  form  of  energy  appears. 

408.  Heat   from    Percussion. — A  small  iron  rod 
placed  upon  an  anvil  may  be  heated  to  redness  by  re- 
peated blows  of  a  hammer.     (Experiment   169.)     The 
energy  of  the  moving  mass  is  broken  up,  so  to  speak,  and 
distributed  among  the  molecules,  producing  the  form  of 
molecular  motion  that  we  call  heat     The  same  trans- 
formation was  illustrated  in  the  kindling  of  a  fire  by  the 
"  flint  and  steel "  of  a  century  ago.    The  bullet  is  heated 
by  its  blow  against  the  target  and  the  water  is  wanner 
at  the  foot  of  Niagara  than  in  the  rapids  above. 


318  NATURAL   PHILOSOPHY.  §  41 1 

409.  Heat   from   Friction. — Common  matches  are 
ignited  and  cold  hands  warmed  by  the  heat  developed 
by  friction.     It  is  said  that  some  savages  kindle  fires  by 
skillfully  rubbing  together  well-chosen  pieces  of  wood. 
A  railway  train  is  really  stopped  by  the  conversion  of  its 
motion  into  heat  by  means  of  the  brakes.    Examples  of 
this  change  are  matters  of  every-day  experience.     (Ex- 
periment 168.) 

410.  Heat  from  Chemical  Action.— When  coal  is 
burned,  the  carbon  and  oxygen  particles  rush  together 
with  tremendous  violence,  energy  of  position  being  con- 
verted into  energy  of  motion. 

The  molecular  motions  produced  by  this  clash- 
ing of  particles  constitute  heat  and  have  a  me- 
chanical value. 

411.  Heating  Powers.— If  a  given  weight  of  carbon 
be  burned,  the  heat  of  the  combustion  would  warm  about 
8000  times  that  weight  of  water  1°  C.    In  like  manner, 
the  combustion  of  a  gram  of  hydrogen  would  yield  more 
than  34000  heat  units  (gram-centigrade). 

(a.)  The  following  table  shows  the  heating  powers  of  several 
substances  when  burned  in  oxygen  : 


Hydrogen  34,462  I  Carbon 

Petroleum 12,300     Alcohol  (C8HSO). 


(6 )  The  heating  powers  mentioned  above  may  be  adapted  to 
Fahrenheit  degrees  by  multiplying  them  respectively  by  £.  As 
they  stand,  the  numbers  represent  the  number  of  times  its  own 
weight  of  water  could  be  wanned  1°  C.  by  burning  the  substance 
in  oxygen. 


§  414  THERMODYNAMICS.  3l9 

412.  First  Law  of  Thermodynamics. —  When 
heat  is  transformed  into  mechanical  energy  or 
mechanical  energy  into  heat,  the  quantity  of 
heat  equals  the  quantity  of  inechanical  energy, 

4I3-  Joule's  Equivalent. — It  is  a  matter  of  great 
importance  to  determine  the  numerical  relation  between 
heat  and  mechanical  energy;  to  find  the  equivalent  of  a 
heat  unit  in  units  of  work.  This  equivalent  was  first  as- 
certained by  Dr.  Joule,  of  Manchester,  England.  His 
experiments  were  equal  in  number  and  variety  to  the 
importance  of  the  subject.  He  showed  that  the  mechan- 
ical value  of  the  heat  required  to  warm  a  given  weight  of 
water — 

I"  C.,  would  lift  the  water J424  meters  against  gravity. 

\  1390  feet 
1°  F.,  would  lift  the  water 772 

Any  weight  unit  may  be  used  without  changing  the 
above  values  which  should  be  remembered. 

Eeferring  to  the  first  iinit  mentioned  in  §  376,  we  say 
that  the  mechanical  value  of  a  heat  unit  is  424  gram- 
meters  or  1390  foot-pounds. 

Referring  to  the  second  unit  there  mentioned,  we  say 
that  the  mechanical  value  of  the  heat  unit  is  772  foot- 
pounds. 

414.  The  Steam-Engine. — The  steam-engine  is  a 
machine  for  utilizing  the  tension  of  steam.  Its  essential 
parts  are  a  boiler  for  the  generation  of  steam  and  a 
cylinder  for  the  application  of  the  tension  to  a  piston. 


320  NATURAL  PHILOSOPHY.  §  4*5 

415.  Double-Acting  Engine. — In  a  double-acting 
steam-engine,  the  steam  is  admitted  to  the  cylinder 
alternately  above  and  below  the  piston.  This  alternate 
admission  of  the  steam  is  accomplished  by  means  of  a 
sliding-valve.  The  sliding-valve  is  placed  in  a  steam- 
chest,  8,  which  is  fastened  to  the  side  of  the  cylinder,  G. 


FIG.  189. 

(?,.)  In  the  figure,  the  steam-chest  is  represented  as  being  placed 
at  a  distance  from  the  cylinder;  this  is  merely  for  the  purpose  of 
making  plain  the  communicating  passages  to  and  from  the  chest. 
Steam  from  the  boiler  enters  at  M,  passes  through  A  to  the  cylin- 
der, where  it  pushes  down  the  piston  as  indicated  by  the  arrows 
in  Fig.  189.  The  steam  below  the  piston  escapes  by  B  and  F. 

(&.)  As  the  piston  nears  the  opening  of  B  in  the  cylinder,  tho 
sliding-valve  is  raised,  by  means  of  the  rod,  R,  to  the  position 
indicated  in  Pig.  ISO.  Steam  now  enters  the  cylinder  by  B  aud 


§  416  THERMODYNAMICS.  321 

pushes  up  the  piston.  The  steam  above  the  piston  escapes  by  A 
and  N.  As  the  piston  neare  the  opening  of  A  in  the  cylinder,  the 
sliding- valve  is  pushed  down  by  R  and  the  process  is  thus  repeated. 


FIG.  190. 

(c.)  The  piston-rod  and  the  sliding-valve  rod  work  through 
s: earn-tight  packing-boxes.  The  piston  is  connected  with  a  crank 
on  the  shaft  of  the  engine,  so  that  the  to-and-fro  motion  of  the 
piston  produces  a  rotary  motion  of  the  shaft.  Smoothness  of 
motion  is  secured  by  attaching  a  heavy  fly-wheel  to  the  shaft  of 
the  engine.  A  Uttle  reflection  will  show  that  the  fly-wheel  also 
acts  as  an  accumulator  of  energy. 

416.  Non-Condensing  Engines. — When  the  steam 
is  forced  out  at  N  (Fig.  190),  it  has  to  overcome  an  at- 
mospheric pressure  of  15  pounds  to  the  square  inch. 
Such  an  engine  is  known  as  a  non-condensing  engine. 
It  may  be  recognized  by  the  escape  of  steam  in  puffs. 


PHiLOSOPBT.  §  416 

It  is  generally  a  high-pressure  engine.    The  railway 
locomotive  is  a  high-pressure,  non-condensing  engine. 

417.  Condensing    Engines. — The  steam  may  be 
conducted  from  the  exhaust  pipe,  N  (Fig.  190).  to  a  cham- 
ber called  a  condenser.     Steam  from  the  cylinder  and  a 
spray  of  cold  water  being  admitted  at  the  same  time,  a 
vacuum  is  formed  and  the  loss  of  energy  due  to  atmos- 
pheric pressure  is  avoided.     Such  an  engine  is  known  as 
a  condensing,  or  low-pressure  engine.     Steamboat  en- 
gines are  generally  condensing  engines. 

(a.)  Low-pressure  engines  are  always  condensing  engines.  A 
low-pressure  engine  will  do  more  work  with  a  given  amount  o* 
fuel  than  a  high-pressure,  non-condensing  engine  will,  is  less  liable 
to  explosion  and  causes  less  wear  and  tear  to  the  machinery.  But 
it  must  be  larger,  more  complicated,  more  costly  and  less  easily 
portable. 

418.  Heat  and  Work  of  Steam-Engines.—  More 
heat  is  carried  to  the  cylinder  of  a  steam-engine  than  ia 
carried  from  it. 

Tlie  piston  does  work  at  every  stroke  and  this 
work  comes  from  the  heat  that  disappears.  Every 
stroke  of  the  piston  transforms  heat  energy  into 
mechanical  energy. 

Careful  experiments  show  that  the  heat  destroyed  and 
the  work  performed  are  in  strict  agreement  with  Joule's 
equivalent.  With  a  given  supply  of  fuel,  the  engine  wih 
give  out  less  heat  when  it  is  made  to  work  haid  than 
when  it  runs  without  doing  much  work. 


§419 


RECAPITULA  TtON. 


419.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


f  DEFINITION. 


MECHANICAL  POWER  CHANGED  TO  HEAT  BY 


(PERCUSSION, 
FRICTION. 


ATOMIC  ATTRACTION. 


JOULE'S  EQUIVALENT. 


STEAM-ENGINES 


A  SOURCE  OK  HEAT. 


IEATING  POWERS  OF  SEVERAL  SUBSTANCES 


ESSBNTIAI  PASTB. 
DOUBLE  ACTING. 
CONDENSING. 


NON-CONI 

R3LATION    BKTWKBN   HEAT  AMD    W  OWt 


824  NATURAL  PHILOSOPHY.  §  419 


EXERCISES. 

1.  If  it  were  possible  to  convert  heat  into  mechanical  power 
without  lobs,  how  far  would  the  heat  giren  out  by  1  gram  of  water 
In  cooling  1°  C.  lift  another  gram  of  water  against  the  force  o? 
gravity  ?    Give  your  answer  in  meters  and  in  feet. 

2.  How  higli  would  it  lift  2  grams  of  water? 

Ans.    212  m.,  or  695  feet. 

3.  How  high  would  it  lift  2  grams  of  lead  ? 

4.  How  high  would  the  heat  thus  given  out  by  a  pound  of 
water  lift  a  pound  weight  ? 

5.  How  high  would  the  heat  thus  given  out  by  10  pounds  of 
water  lift  a  5  pound  weight  ?  Ans.    848  m.,  or  2780  feet. 

6.  How  high  would  the  heat  thus  given  out  by  5  pounds  of 
water  lift  a  10  pound  weight? 

7.  If  a  pound  of  water  falls  772  feet,  how  much  will  it  be  heated 
by  the  s'opping  of  its  motion  ? 

8.  If  a  2  pound  weight  falls  772  feet,  how  much  heat  will  be 
developed  by  the  percussion  when  it  strikes  the  ground  ? 

9.  How  many  heat  units  (gram-centigrade)  must  be  transformed 
In  order  to  raise  1000  grams  to  a  height  of  424  meters  ? 

10.  How  many,  thus  to  raise  2  kilograms?  Ans.     2000. 

11.  How  many  grams  of  water  can  be  heated  from  the  freezing 
to  the  boiling  temperature  by  the  heat  produced  by  burning  1 
gram  of  hydrogen  ? 

12.  How  many  weights  of  water  may  be  thus  heated  by  burn- 
ing 1  weight  of  pure  coal  (carbon)?  Ans.    80.8. 

13.  If  a  good  steam-engine  utilizes  only  about  10  per  cent,  of  the 
heat  energy  of  its  fuel,  how  many  foot  pounds  of  work  can  it  do 
with  a  ton  of  coal  that  is  assumed  to  be  pure  carbon  ? 

Ans.    2000  x  8080  x  1390  x  TV 

14  Would  it  do  more  work  or  less  with  the  burning  of  a  like 
weight  of  petroleum  ? 

15.  Can  heat  be  destroyed?    Can  energy? 


§  419  REVIEW   QUESTIONS.  325 

REVIEW     QUESTIONS. 

1.  What  three  things  determine  the  rapidity  of  vibration  of  a 
string  ? 

2.  (a.)  Will  sound  waves  pass  through  water?    (b.)  Through  a 
vacuum  ? 

8.  What  is  in  the  upper  part  of  a  thermometer  tube  ? 

4.  Upon  what  does  the  loudness  of  sound  depend  ? 

5.  Why  are  spaces  left  between  the  ends  of  the  rails  in  laying 
railway  tracks? 

6.  Why  is  a  gallon  of  alcohol  worth  more  in  cold  than  in  warm 
weather  ? 

7.  Will  a  siphon  work  in  a  vacuum  ?    Wliy  ? 

8.  Can  you  pump  any  water  out  of  an  air-tight  cistern  full  of 
water?    Explain. 

9.  Can  you  pump  any  water  out  of  an  air-tight  cistern  half  full 
of  water?    Explain. 

10.  Construct  a  simple  piece  of  apparatus  to  illustrate  your 
answers  to  the  last  two  questions. 

11.  Which  seems  colder,  a  wiudy  or  a  still  day,  the  temperature 
being  the  same  1    Why  ? 

12.  If  there  were  no  water  on  the  earth,  would  the  difference* 
in  temperature  between  day  and  night  and  between  summer  and 
winter  be  greater  or  less  than  they  now  are?    Why  ? 

13.  Does  heat  always  expand  water? 

14.  How   can    you   heat 
water  above  212  F.  ? 

15.  Do  all  solids  and  gasea 
expand  equally  ? 

16.  Show  that  the  appa. 
ratus  represented  in  Fig.  191 
is  a  lever  and  tell  of  what 

PlO.  191.  class.    Indicate  the  positions 

for  Pf  W  and  F. 

17.  How  does  perspiration  increase  one's  personal  comfort  iu 
warm  weather? 


326 


NATURAL  PHILOSOPHY. 


§419 


18.  If  a  pound  of  water  at  100°  C.  be  added  to  a  pound  of  watei 
at  0°  C.,  what  will  be  the  resulting  temperature? 

19.  Fig.  192  represents  a  "  dropping  bottle."    (a.)  Why  does  ail 
bubble  up  from  the  lower  end  of  tube  a 

when  liquid  drops  from  the  lower  end  of  c  t 
(6.)  Why  does  the  liquid  cease  to  drop  when 
the  finger  closes  the  tube  at  a  ? 

20.  The  "  return  ball "  is  a  common  toy 
made  by  fastening  a  wooden  ball  to  the  end 
of  an  elastic  cord.    When  thrown  from  the 
hand,  the  cord  being  held  by  the  free  end, 
the  bail  returns  and  is  caught  in  the  hand. 
Does  the  motion  of  the  ball  resemble  water 
waves  or  sound  waves  the  more  closely? 

21.  A  centigrade  thermometer  records  33°.      What  will  be 
the  reading  of  a  Fahrenheit  thermometer  under  similar  circum- 


FIG.  192. 


22.  Show  haw  the  apparatus  represented  in  Fig.  193  acts  to 
obviate  the  trouble  arising  from 
the  "  boiling  away  "  of  the  water 
in  the  basin. 

23.  Describe  the  suction 
pump. 

24  What  is  the  cause  of  dif- 
ference of  pitch  ? 

25.  What   is    meant   by  the 
temperature  of    the    maximum 
density  of  water? 

26.  What  is  meant  by  the  ab- 
solute zero  of  temperature  ? 

27.  What  is  the  ordinary  me- 
dium for  the  transmission  (a.)  of 
sound  ?    (6.)  Of  radiant  heat  ? 

28.  Give  the  rule  for  finding 
the     downward     pressure     of 

FIG.  193.  liquids  caused  by  gravity. 


§419 


REVIEW  QUESTIONS. 


327 


29.  What  are  ions? 

30.  A  bullet  is  thrown  downward 
with  a  velocity  of   20  meters  per 
second.     What  will  be  its  velocity 
at  the  end  of  4  seconds  ? 

31.  Fill   with  water  a  cup  sus- 
pended by  three  cords.    Turn  the 
cup  round  and  rouud,  ti^us  twisting 
together  the  cords.     Leave  the  cup 
free  to  untwist  the  cords  and  water 
will  fly  from  the  edges  of  the  cup  as 
shown  in  Fig.  194.     What  physical 
law  is  thereby  illustrated  ? 

32.  Note  the  temperature  by  the 
school-room    thermometer.      What 
is  the  velocity  of  sound  now  and 
here? 

33.  Define  chemical  change  and 
give  an  illustration  thereof. 


FIG.  194 


CHAPTEB   IX. 

LIGHT. 

SECTION      I. 

THE  NATURE,  VELOCITY  AND  INTENSITY  OF  LIGHT. 

420.  What  is    Light  ? — Light   is   the   mode  of 
motion  that  is  capable  of  affecting  the  optic  nerve. 
It  is  physically  identical  with  radiant  heat,  the 
only  difference,  in    any    case,   being    one    of  wave 
length.     (See  §  397.) 

(a.)  We  have  seen  that  the  vibrations  of  air  particles  in  a  sound 
wave  are  to  and  fro  in  the  line  of  propagation.  In  the  case  of  ra- 
diant heat  and  light,  the  ether  particles  vibrate  to  and  fro  across  the 
line  of  propagation.  Vibrations  in  a  sound  wave  are  longitudinal; 
those  of  a  heat  or  light  wave  are  transversal. 

421.  Luminous  and   Non-Luminous   Bodies.— 

Bodies  that  emit  light  of  their  own  generating,  as  the 
sun  or  a  candle,  are  called  luminous.  Bodies  that  merely 
diffuse  the  light  that  they  receive  from  other  bodies  are 
said  to  be  non-luminous  or  illuminated.  Trees  and 
plants  are  non-luminous. 

422.  Transparent,    Translucent    and    Opaque 
Bodies. — Transparent  bodies   allow  objects  to  be  seen 
distinctly  through  them,  e.  q.,  air,  glass  and  water. 


424 


MATURE   OF   LIGHT. 


329 


Translucent  bodies  transmit  light  but  do  not  allow 
bodies  to  be  seen  distinctly  through  them,  e.  g.,  ground 
glass  and  oiled  paper. 

Opaque  bodies  cut  off  the  light  entirely  and  prevent 
objects  from  being  seen  through  them  at  all. 

423.  Luminous  Rays. — A  single  line  of  light  is 
called  a  ray.     Tlie  ray  may,  without  considerable 
error,  be  deemed  the  path  of  the  wave. 

424.  Luminous   Beams  and   Pencils. — A  collec- 
tion of  parallel  rays  constitutes  a  beam;  a  cone  of  rays 
constitutes  a  pencil.     The  pencil  may  be  converging  or 
diverging. 


FIG.  195. 

Experiment  207.— Provide  two  or  three  perforated  screens  and 
arrange  them  as  shown  in  Fig.  195,  so  that  the  holes  and  a  candle 
game  shall  be  in  the  saine  straight  line.  When  the  eye  is  placed 


330  NATURAL    PXILOSOPHT.  §425 

in  this  line  behind  the  screens,  light  passes  from  the  flame  to  the 
eye ;  the  flame  is  visible.  A  slight  displacement  upward,  down- 
ward or  sidewise  of  the  eye,  the  flame  or  any  screen,  cuts  off  the 
light  and  renders  the  flame  invisible. 

Make  the  screen  as  follows:  Prepare  a  piece  of  wood,  I£x2| 
x  18  inches,  taking  care  that  the  edges  are  square.  Saw  it  into 
six  pieces,  each  three  inches  long.  Prepare  three  pieces  of  wood, 
3  x  4  x  £  inches.  Place  three  postal  cards  one  over  the  other  on  a 
board,  and  pierce  them  with  a  fine  awl  or  stout  needle,  ^  inch  from 
the  end  and  1£  inch  from  either  side  of  the  card.  With  a  sharp 
knife  pare  off  the  rough  edges  of  the  holes,  and  pass  the  needle 
through  each  hole  to  make  the  edges  smooth  and  even.  Over  the 
^  x  3  inch  surface  of  one  of  the  blocks,  place  the  unperforated  end 
of  one  of  the  postal  cards  and  over  this  place  one  of  the  3x4  inch 
pieces,  so  that  their  lower  edges  shall  be  even.  Tack  them  in 
this  position.  Make  thus  two  similar  screens.  The  three  screens, 
with  a  bit  of  candle  three  inches  long,  piaced  upon  one  of  the 
remaining  blocks,  furnishes  the  material  for  the  experiment 
above.  Save  the  screens  and  three  blocks  for  future  use.  (See 
Pig.  200.) 

425.  Rectilinear  Motion  of  Light.— A  medium 
is  homogeneous  when  it  has  uniform  composition  and 
density.  In  a  homogeneous  medium,  light  travels 
in  straight  lines. 

(a)  The  familiar  experiment  of  "taking  sight"  depends  upon 
this  fact,  for  we  see  objects  by  the  light  which  they  send  to  ifie  eye. 
We  cannot  see  around  a  corner  or  through  a  crooked  tube. 

Experiment  208.— Place  a  lighted  candle  about  a  yard  from  a 
white  screen  iu  a  darkened  room.  (The  wall  of  the  room  may 
answer  for  the  screen.)  Pierce  a  large  pin-hole  in  a  card  and 
hold  it  between  the  flame  and  the  screen.  An  inverted  image  of 
the  flame  will  be  found  upon  the  screen, 


§426 


NATURE   OF   LIGHT. 


331 


Experiment  209.— Cover  one  end  of  a  tube,  10  or  12  cm.  long, 
with  tinfoil  ;  the  other  end  with  oiled  paper.  Prick  a  pin-hole  in 
the  tinfoil  and  turn  it  toward  a  candle  flame.  An  inverted 
image  may  be  seen  upon  the  oiled  paper.  The  size  of  the 
image  will  depend  upon  the  distance  of  the  flame  from  the 
pin-hole. 

This  apparatus  rudely  represents  the  eye  (§  473),  the  pin-hole 
corresponding  to  the  pupil  and  the  oiled  paper  to  the  retina.  Al- 
most any  housekeeper  will  give  you  an  empty  tin  can.  Place  it 
upon  a  hot  stove  just  long  enough  to  melt  off  one  end,  thrust  a 
stout  nail  through  the  centre  of  the  other  end,  cover  the  nail-hole 
with  tinfoil  and  you  will  have  the  greater  part  of  the  apparatus. 

426.  Inverted  Images. — If  light  from  a  highly- 
illuminated  body  be  admitted  to  a  darkened  room  through 
a  small  hole  in  the  shutter  and  there  received  upon  a 
white  screen,  it  will  form  an  inverted  image  of  the  ob- 


ject upon  the  screen.  Every  visible  point  of  the  illumi- 
nated object  sends  a  ray  of  light  to  the  screen.  Each 
ray  brings  the  color  of  the  point  which  sends  it  and 
yrints  the  color  upon  the  screen.  As  the  rays  are 


NATURAL  PHILOSOPHY. 


§425 


straight  lines,  they  cross  at  the  aperture;  hence,  the  in- 
version of  the  image.  The  image  will  be  distorted  un- 
less the  screen  be  perpendicular  to  the  rays.  The  dark- 
ened room  constitutes  a  camera  obscura.  The  image  of 
the  school  playground  at  recess  is  very  interesting  and 
easily  produced. 

427.  Velocity  of  Light. — Light  moves  with  a 
velocity  of  about  186,000  miles  per  second. 

(a.)  It  would  require  more  than  17  years  for  a  rannon-ball  to 
pass  over  the  distance  between  the  sun  and  the  earth  ;  light  makes 
the  journey  in  8  min.  18  sec.  For  the  swiftest  bird  to  pass  around 
the  earth  would  require  three  weeks  of  continual  flight ;  light 
goes  as  far  in  less  than  one  seventh  of  a  second.  For  terrestrial 
distances,  the  passage  of  light  is  practically  instantaneous  (%  325). 


36' 


FIG.  197. 


Experiment  210.— Let  a  candle  or  lamp  at  L,  (Fig.  197)  be  the 
source  of  light  ;  8,  a  cardboard  screen  with  a  small  perforation  at 
the  level  of  the  flame  ;  A,  a  screen  one  inch  square  and  a  foot  from 
S;  B,  a  screen  two  inches  square,  two  feet  from  8;  C.  a  screen 
three  inches  square,  three  feet  from  S.  It  will  easily  be  seen  thai 
A  will  cut  off  all  the  light  from  B  and  C.  If  A  be  removed,  ti,e 
quantity  of  light  which  it  received,  no  more  and  no  less,  will  fall 
upon  B.  If  now  B  be  removed,  the  quantity  of  light  which  pre« 
riously  illuminated  A  and  B  wjlj  fall  upon  Q 


§428 


NATURti   OF  LIGHT, 


333 


We  thus  see  the  same  quantity  of  light  successively  illuminating 
one,  four  and  nine  square  inches.  *  One  square  inch  at  B  will  re- 
-eive  one-fourth,  and  one  square  inch  at  C will  receive  one-ninth  as 
much  light  as  one  square  inch  at  A.  The  light  being  spread  over 
*  greater  surface  is  correspondingly  diminished  in  intensity. 

428.  Effect  of  Distance  upon  Intensity.— The 
intensity  of  light  received  bij  an  illuminated  body 
caries  inversely  as  the  square  of  its  distance  from 
the  source  of  light. 


FIG.  198. 


(a.)  This  principle  is  applied  in  photometry  or  the  measurement 
of  light.  A  simple  photometer  is  represented  in  Fig.  198.  S  is  a 
screen  of  white  paper  or  cardboard ;  R  is  a  small  rod  placed  up- 
right a  few  inches  from  S  (a  cheap  j  en  and  pen-holder  or  a  lead 
penc  1  held  by  a  bit  of  wax  on  the  table  will  answer).  The  flame 
of  the  candle  and  that  of  the  lamp  should  be  at  the  same  level ;  the 
flat  lamp-wick  should  stand  diagonally  to  the  screen. 

Place  the  candle  about  20  inches  from  8  and  move  L  about  until 
the  two  shadows  upon  S  just  touch  and  are  of  equal  darkness. 
The  candle  and  the  lamp  are  now  throwing  equal  amounts  of  light 
upon  H. 


PHILOSOPHY.  §  429 

If  the  distance  from  8  to  L  be  twice  that  from  8  to  C,  then  S  is 
four  times  as  powerful  a  light  as'C;  if  the  distance  be  three  times 
as  far,  L  is  nine  times  as  powerful.  If  C  be  20  inches  from  <S  and 

L  be  65  inches,  then  L  is          ~     times  as  powerful  as  C. 


(b.)  Another  method  is  to  use  a  screen  of  paper  with  a  grease 
spot  in  its  centre.  If  this  spot  be  viewed  from  the  side  toward  the 
light,  it  will  appear  darker  than  the  rest  of  the  screen  ;  if  viewed 
from  the  other  side,  it  will  appear  brighter.  Place  the  two  lights 
to  be  compared  on  opposite  sides  of  the  screen  so  that  they  shall 
be  in  the  same  straight  line  with  the  centre  of  the  grease  spot. 

Move  one  light  toward  or  from  the  screen  until  the  grease  spot 
is  invisible  ;  the  two  sides  of  the  screen  are  then  equally  illumi- 
nated. 

Find  the  distance  of  each  light  from  the  screen  ;  square  it  ;  di- 
vide  the  greater  square  by  the  less  :  the  quotient  will  be  the  ratio 
between  the  intensities  of  the  two  lights. 

NOTE-  —  In  either  of  these  methods  of  photometry  (light  meas- 
uring), there  should  be  no  light  falling  on  the  screen  except  whaf 
comes  from  the  two  lights  being  compared. 

429.  Recapitulation.—  To  be  amplified  by  the  pupil 
for  review. 


LIGHT 


DEFINITION. 
TRANSPARENCY  OF 

LUMINOUS             .... 

BODIES. 
f  BODY. 
1RAY. 

BEAM. 

RIGHT   LINE  MOTIOI 
INVERTED   IMAGES. 
VELOCITY. 
INTENSITY. 

PENCIL. 
I, 

§  429  EXERCISES.  335 


EXERCISES. 

1.  Do  waves  of  light  resemble  sound  waves  more  or  less  than 
they  do  water  waves  ?     Explain. 

2.  How  does  light  differ  from  radiant  heat? 

3.  What  is  a  sunbeam  ? 

4.  Explain  the  formation  of  inverted  images. 

5.  If  the  sun  were  blotted  out  of  existence,  how  long  would  it 
be  before  the  earth  would  be  wrapped  in  darkness  ? 

6.  If  the  moon  were  twice  as  far  from  the  earth  as  it  is,  what 
would  be  the  effect  upon  the  brilliancy  of  moonlight  ? 

7.  An  electric  lamp  100  ft.  north  of  me  and  one  200  feet  south 
of  me  illuminate  opposite  sides  of  a  sheet  of  paper  in  my  hand 
and  render  invisible  a  grease  spot  on  the  paper. 

(a.)  Which  lamp  is  giving  the  more  light  to  the  paper  ? 
(6.)  How  do  the  luminous  powers  of  the  lamps  compare  1 

8.  What  is  the  difference  between  a  luminous  and  an  illumi- 
nated body  ? 

9.  An  opaque  screen,  3  inches  square,  is  held  12  inches  in  front 
of  one  eye  ;  the  other  eye  is  shut ;  the  screen  is  parallel  with  a 
wall  100  feet  distant.     What  area  on  the  wall  may  be  concealed 
by  the  screen  ? 

10.  A  "  standard  "  candle  (burning  120  grains  of  sperm  per 
hour)  is  2  feet  from  a  wall,  a  lamp  is  6  feet  from  the  wall.     They 
cast  shadows  of  equal  intensity  on  the  wall.     What  is  the  "  candle 
power  "  of  the  lamp  ? 


SECTION   1 1 . 

REFLECTION  OF  LIGHT. 

Note.— The  hx-li..sut,  or  poi-te-iumiere,  is  composed  of  one  or 
more  mirrors,  by  means  of  which  a  beam  of  light  may  be  thrown 
in  any  desired  direction.  The  instrument  may  be  had  of  appara- 
tus manufacturers  at  prices  ranging  from  $12  upward.  Direc- 
tions for  making  one  may  be  found  in  Mayer  &  Barnard's  little 
book  on  "Light." 

43O.  Reflection.— If  a  sunbeam  enter  a  darkened 
room  by  a  hole  in  the  shutter,  as  at  A,  and  fall  upon  s> 


PIQ.  199. 

polished  plane  surface,  as  at  B,  it  will  be  continued  in 
a  different  direction,  as  toward  C.  AB  is  called  the 
incident  beam  and  BC,  the  reflected  beam.  The 


§  43^  REFLECTION  OF  LIGHT.  337 

incident  and  the  reflected  beams  are  in  the  same  modium, 
the  air. 

A  change  in  the  direction  of  light  without  a, 
cJtange  in  its  medium  is  called  reflection  of 
light. 

Experiment  211.  —  Place  two  of  thesceens  and  the  three 
extra  blocks  mentioned  in  Experiment  297  in  position,  as  shown 
in  Fig.  200.  At  the  middle  of  the  middle  block,  place  a  bit  of 
win-low  glass,  painted  on  the  underside  with  black  varnish.  Ou 
the  blocks  that  carry  the  screens,  place  bits  of  glass,  n  and  o,  of 
the  same  thickness  as  the  black  mirror  on  the  middle  block. 


FIG.  200. 

Place  a  candle  flame  near  the  hole  in  one  of  the  screens,  as 
shown  in  the  figure.  Light  from  the  candle  will  pass  through  A, 
be  reflected  at  m  and  pass  through  B.  Place  the  eye  in  such  a 
position  that  the  spot  of  light  in  the  mirror  may  be  seen  through 
Ji.  Mark  the  exact  spot  in  the  mirror  with  a  needle  held  in  place 
by  a  bit  of  wax. 

Phice  a  piece  of  stiff  writing  paper  upright  upon  m  and  n.  mark 
the  position  of  B  and  of  m,  and  draw  on  the  paper  a  straight  line 
joining  these  two  points.  The  angle  between  this  line  and  the 
lower  edge  of  the  paper  coincides  with  the  angle  Bmn.  Reverse 
*}ie  paper,  placing  it  upon  m  and  o.  It  will  be  found  that  the  same 
angle  coincides  with  Amo.  Amo  and  Bmn  being  thus  equal, 
the  angle  of  incidence  equals  the  angle  of  reflection.  (§  57.) 
15 


338  frATfrSAL  ffflLOSOPHf.  §  43! 

Experiment  212. — Get  a  piece  of  looking  glass,  about  an  inch 
square.  On  the  back  of  this  little  mirror,  at  the  edges  and  in  a 
triangular  position,  place  three  bits  of  soft  wax  each  the  size  of  a 
pea.  Place  the  mirror  on  the  wrist  with  one  of  the  wax  supports 
on-  the  pulse. 

Let  this  mirror  replace  the  mirror  shown  in  Fig.  199,  hold  the 
arm  steady  and  watch  the  motions  of  the  sun  spot  on  the  wall  or 
ceiling.  They  are  like  the  beatings  of  the  pulse  which  they 
make  visible  to  the  whole  class. 

The  reflected  beam  moves  through  an  angle  twice  as  great  as 
does  the  mirror.  If  the  pupil  trying  the  experiment  laughs,  or 
becomes  excited  in  any  way,  the  change  in  the  movement  of  the 
pulse  becomes  evident  to  all  in  the  room. 

431.  Law  of  Reflection. — The  reflection  of  light 
from  a  polished  surf  ace  obeys  the  now  familiar  law  :  The 
angle  of  incidence  is  equal  to  the  angle  of  reflec- 
tion. 

Experiment  213.— Let  a  beam  of  light  fall  upon  a  sheet  of 
drawing  paper  in  a  darkened  room  ;  it  will  be  scattered  and  illu- 
minate the  room.  Let  it  fall  upon  a  mirror ;  nearly  all  of  it  will 
be  reflected  in  a  definite  direction  and  intensely  illuminate  only  a 
part  of  the  room. 

Experiment  214. — Place,  side  by  side  upon  a  board,  a  piece  of 
black  cloth  (not  glossy),  a  piece  of  drawing  paper  and  a  piece  of 
looking-glass.  Allow  a  beam  of  sunlight  to  fall  upon  the  cloth  and 
notice  the  absorption.  Let  it  fall  upon  therpaper,  and  notice  the 
diffusion  of  the  light  and  its  effects.  Let  it  fall  upon  the  look- 
ing-glass, and  notice  the  regular  reflection  and  its  effects.  Move 
the  board  so  that  the  cloth,  paper  and  glass  shall  pass  through 
the  beam  in  quick  succession  and  notice  the  effects. 

Experiment  215.— In  the  darkened  room,  place  a  tumbler  of 
water  upon  a  table  ;  with  a  hand  mirr.,r,  reflect  me  sunbeam  down 


§  433  SEfLECTlOtf  OP  LIGffT.  $3$ 

into  the  water  ;  the  tumbler  will  be  visible.  Stir  a  teaspoonful  of 
milk  into  the  water  and  again  reflect  the  sunbeam  into  the  liquid  ; 
the  whole  room  will  be  illuminated  by  the  diffused  light,  the 
tumbler  of  milky  water  acting  like  a  luminous  body. 


432.  Diffused  Light. — Light  falling  upon  an  opaque 
body  is  generally  divided  into  three  parts :  the  first  is 
regularly  reflected  in  obedience  to  the  law  above  given  ; 
the  second  is  irregularly  reflected  or  diffused ;  the  third 
is  absorbed. 

The  irregular  reflection  is  due  to  the  fact  that  the 
bodies  are  not  perfectly  smooth,  but  present  little  pro- 
tuberances that  scatter  the  light  in  all  directions  and 
thus  render  them  visible  from  any  position.  (Fig.  239. ) 

Light  regularly  reflected  gives  an  image  of  the  body 
from  which  it  came  before  reflection ;  light  irregularly 
reflected  gives  an  image  of  the  body  that  diffuses  it.  A 
perfect  mirror  would  be  invisible. 

Luminous  bodies  are  visible  on  account  of  the 
light  that  they  emit;  non-luminous  bodies  are 
visible  on  account  of  the  light  that  they  diffuse. 

433-  Apparent  Direction  of  Bodies. — Every  point 
of  a  visible  object  sends  a  cone  of  rays  to  the  eye.  The 
pupil  of  the  eye  is  the  base  of  the  cone.  TJie  point  al- 
ways appears  at  the  place  where  these  rays  seem 
to  intersect  (i.  e.,  at  the  real  or  apparent  apex  of  the 
cone). 

If  the  rays  pass  in  straight  lines  from  the  point  to  the 
eye,  the  apparent  position  of  the  point  is  its  real  po«i- 


340  NATUItAL  PHILOSOPHY.  §  433 

tion.  If  these  rays  be  bent  by  reflection  or  in  any  other 
manner,  the  point  will  appear  to  be  in  the  direc- 
tion of  the  rays  as  they  enter  the  eye.  No  matter 
how  devious  the  path  of  the  rays  in  coming  from  the 
point  to  the  eye,  this  rule  holds  good. 

Experiment  216. — Hold  a  printed  page  before  a  common  mirror. 
Notice  that  each  printed  line  is  reversed ;  each  letter  is  turned  side 
for  side.  "Look  at  yourself  "  in  the  mirror.  The  right  hand  of 
the  image  is  opposite  your  left  hand  just  as  it  would  be  if  the  image 
were  another  person  facing  you.  This  reversing  effect  is  called 
lateral  inversion. 

Experiment  217.— Place  a  jar  of  water  10  or  15  cm.  back  of  a 
pane  of  glass  placed  upright  on  a  table  in  a  dark  room.  Hold  a 
lighted  candle  at  the  same  distance  in  front  of  the  glass.  The  jar 
will  be  seen  by  light  transmitted  through  the  glass.  An  imago 
of  the  candle  will  be  formed  by  light  reflected  by  the  glass.  The 
image  of  the  candle  will  be  seen  in  the  jar,  giving  the  ap- 
pearance of  a  candle  burning  in  water.  The  same  effect  may 
be  produced  in  the  evening  by  partly  raising  a  window  and  holding 
the  jar  on  the  outside  and  the  candle  ou  the  inside. 

434.  Plane  Mirrors  ;  Virtual  Images.— If  an  ob- 
ject be  placed  before  a  mirror,  an  image  of  it  appears  be- 
hind the  mirror.  This  is  called  a  virtual  image.  All 
virtual  images  are  optical  illusions  and  are  to  be  clearly 
distinguished  from  the  real  images  to  be  studied  soon. 

Each  point  of  the  image  will  seem  to  be  as  far 
behind  the  mirror  as  the  corresponding  point 
of  the  object  is  in  front  of  the  mirror,  llence,  images 
seen  in  still,  clear  water  are  inverted. 


§435 


REFLECTION  OF  LIGHT. 


341 


(a.)  In  Fig.  201,  rays  of 
light  from  A  are  reflected 
at  B  and  B'  and  enter  the 
eye  fit  7).  These  r;i  ys  HUH  ply 
1 1 />/'€(!>•  1o  converge  at  A', 
\vhidi  is,  therefore,  called 
the  rirtiiiil  image  of  A. 

435-  Concave 
Mirrors. — A  spheri- 
cal, concave  mirror 
may  be  considered  as 
a  small  part  of  a  spher- 
ical shell  with  its  inner  surface  highly  polished.  Let  MN 
(Fig.  202)  represent  the  section  of  such  a  concave,  spheri- 
cal mirror  and  C,  the  centre  of  the  corresponding  sphere. 
C  is  called  the  centre  of  curvature ;  A  is  the  centre  ot 


FIG.  201. 


FIG.  202. 


the  mirror.  A  straight  line  drawn  from  A  through  C, 
as  A  CX,  is  called  the  principal  axis  of  the  mirror.  A 
straight  line  drawn  from  any  other  point  of  the  mirror 
through  C,  as  JCd,  is  called  a  secondary  axis.  The 
point,  F,  midway  between  A  and  C,  is  called  the  princi- 
pal focus.  The  distance,  AF,  is  the  focal  distance  of  the 


342  NATURAL   PHILOSOPHY.  §435 

mirror;  the  focal  distance  is,  therefore,  one-half  the 
radius  of  curvature.  The  angle,  MCN,  is  called  the 
aperture  of  the  mirror. 

(«.)  It  should  be  borne  in  mind  that  radii  drawn  from  G  to  points 
in  the  mirror,  as  /  and  J,  are  perpendicular  to  the  mirror  at  these 
points.  Thus,  the  angles  of  incidence  aud  of  reflection  for  any  ray 
may  be  easily  determined. 

436.  Effect  of  Concave  Mirrors. — The  tendency 
of  a  concave  mirror   is  to   make   incident   rays 
converge  more  or  diverge  less. 

437.  The  Principal  Focus.— A  focus  is  the  point 
toward  which  rays  converge.     All  incident  rays  parallel 
to  the  principal  axis  of  a  concave  mirror  will,  after  re- 
flection, converge  at  the  principal  focus.     The  princi- 
pal focus  is  the  focus  of  rays  parallel  to  the  prin- 
cipal axis. 

The  rays  will  be  practically  parallel  when  their  source 
is  at  a  very  great  distance,  e.g.,  the  sun's  rays.  Solar 
rays  coming  to  the  earth  do  not  diverge  a  thousandth  of 
in  inch  in  a  thousand  miles. 

438.  Conjugate  Foci. — Rays  diverging  from  a  lumi- 
nous point  in  front  of  a  concave,  spherical  mirror  and  at 
a  distance  from  the  mirror  greater  than  its  focal  distance, 
will  converge,  after  reflection,  at  another  point.     Kays 
diverging  from  B  will  form  a  focus  at  b.     (Fig.  203.) 
Kays  diverging  from  b  would  form  a  focus  at  B.     Two 
such  points  a.re  called  conjugate  foci. 


§  439  REFLECTION  OF  LIGHT.  343 

Conjugate  foci  are  two  points  so  related  that 
each  forms  the  ima&e  of  the  other. 


FIG.  203. 

Experiment  218. — In  a  dark  room,  hold  a  candle  between  the 
eye  and  the  concave  side  of  a  bright  silver  spoon  held  a  little  ways 
in  front  of  the  face.  Notice  that  the  inverted  image  of  the 
flame  is  in  front  of  the  spoon. 

Place  the  spoon  between  the  flame  and  your  face  but  so  as  to 
allow  the  face  to  be  illuminated  by  the  candle.  Notice  the  image 
of  the  observer. 

439.  Projection  of  Real  Images  by  Concave 
Mirrors. — The  real  image  formed  by  a  concave  mirror 
may  be  rendered  visible  by  projecting  it  upon  a  screen. 
In  a  darkened  room,  let  a  candle  flame  be  placed  in  front 
of  a  concave  mirror,  at  a  distance  from  it  greater  than 
the  focal  distance  and  less  than  the  radius  of  curvature 
of  the  mirror.  Incline  the  mirror  so  that  the  flame 
shall  not  be  on  the  principal  axis.  Place  a  paper  screen 
at  the  conjugate  focus  of  any  point  in  the  luminous  ob- 
ject. The  proper  position  for  the  screen  may  easily  be 
found  by  trial.  Shield  the  screen  from  the  direct,  rays 
of  the  flame  by  a  card  painted  black.  The  inverted 
image  may  be  seen  by  a  large  class,  (Fig.  204.) 


344 


XA  Tl'L'A  L  PHIL  OSOPHY. 


439 


FIG.  204 

If  the  image  fall  between  the  mirror  and  the  candle,  as 
it  will  if  the  candle  be  at  a  distance  from  the  mirror 
greater  than  the  radius  of  curvature,  the  screen  should 
be  quite  small.  The  image  of  any  powerfully  illuminated 
object  may  thus  be  produced,  as  shown  in  Fig.  205. 

In  both  of  these  cases,  the  reflected  rays  actually  con- 
verge on  the  screen ;  these  images  are,  therefore,  called 
real  images  to  distinguish  them  from  virtual  images. 

Experiment  219.— Hold  the  convex  side  of  a  bright  silver 
spoon  toward  you  and  bring  the  spoon  and  a  candle  into  the 
positions  described  in  Experiment  218.  Notice  that  the  erect 
image  of  the  flame  is  back  of  the  spoon. 

Place  the  spoon  between  the  flame  and  your  face  but  so  as  to 
allow  the  face  to  be  illuminated  by  the  candle.  Notice  the  image 
of  the  observer, 


§441 


REFLECTION   OF  LIGHT. 


345 


FIG.  20.'). 

440.  Convex    Mirrors. — In   convex    mirrors,    the 
images  are  virtual,  erect  and  smaller  than  their  objects. 

441.  Recapitulation.— To  be  amplified  by  the  pupil 
for  review. 


DEFINITION.  f  Plane. 

MIRRORS. -I  Co 
LAW. 


<?/  Curvature. 
j  Principal. 
\  Secondary. 


REGULAR.. 


IRREGULAR. 


f  Definition. 

Foci  ....  -I    Virtual. 

j    Principal. 


I-  Conjugate. 
(  Real. 

I    Virtual. 
IMAGES..-!    ,, 

I  Erect- 
(.  Inverted. 
APPARENT  DIRECTION  OF  OBJECTS. 


346  NATURAL  PHILOSOPHY,  §  44! 


EXERCISES. 

1.  Given  three  points,  A,  B  and  C,  not  in  a  straight  line.     Show, 
by  a  diagram,  how  you  would  place  a  plane  mirror  at  C  so  that 
light  proceeding  from  A  shall  be  reflected  to  B. 

2.  If  you  hold  a  sheet  of  paper  with  a  greased  spot  on  it  be- 
tween you  and  the  light,  the  spot  will  look  lighter  than  the  rest 
of  the  sheet.     Why  is  this  ? 

3.  If  you  hold  the  sheet  in  front  of  you  when  you  are  turned 
away  from  the  light,  the  spot  will  look  darker  than  the  rest  of  the 
sheet.     Why  is  this? 

4.  (a.)  Is  the  image  formed  by  a  convex  mirror  real  or  virtual  ? 
(b.)  Is  it  erect  or  inverted?    (c.)  Is  it  larger  or  smaller  than  the 
object  ? 

5.  What  is  the  difference    between  real   and  virtual  fofc    or 
images  ? 

6.  Why  are  the  images  seen  in  a  pond  of  water  invert**!  * 


SECTION      III. 

REFRACTION  OF  LIGHT. 

Experiment  220.— Procure  a  clear  glass  bottle  with  flat  sHes. 
about  4  inches  (10  cm.)  hroad.  On  one  side,  paste  a  piece  of  pawr, 
in  which  a  circular  hole  has  been  cut.  On  this  clear  space,  draw 
two  ink-marks  at  right  angles  to  each  other,  as  shown  in  Fig.  206. 


FIG.  206. 


Fill  the  bottle  with  clear  water  up  to  the  level  of  the  horizonta\ 
ink-mark.  Hold  it  so  that  a  sunbeam  coming  through  a  hole  in 
the  shutter  of  a  darkened  room  may  pass  through  the  clear  sides 
of  the  bottle  above  the  water  and  notice  that  the  beam  passes 


348  NATURAL  PHILOSOPHY.  §  441 

through  the  bottle  in  a  straight  line.  Raise  the  bottle  so  that  the 
beam  shall  pass  through  the  water  and  notice  that  the  beam  is 
still  straight. 

In  a  card,  cut  a  slit  about  5  cm.  long  and  1  mm.  wide.  Place 
the  card  against  the  bottle  as  shown  in  Fig.  206.  With  a  mirror, 
reflect  the  beam  through  this  slit  at  S,  so  that  it  shall  fall  upon 
the  surface  of  the  water  at  i,  the  int3rsection  of  the  two  ink  marks 
Notice  that  the  reflected  beam  is  straight  until  it  reaches  the 
water  but  that  it  is  bent  as  it  enters  the  liquid. 

Experiment  221.— Put  a  small  coin  into  a  tin  cup  and  place  the 
cup  so  that  its  edge  just  keeps  you  from  seeing  the  coin.  A  ray  of 
light  coming  from  the  coin  toward  you  must  pass  above  the  eye 
and  thus  be  lost  to  sight.  Pour  water  into  the  cup  and  the  coin 
will  become  visible.  The  rays  are  bent  down  as  they  leave  the 
water  and  some  of  them  enter  the  eye. 


FIG.  207. 


Experiment  222. — Notice  that  an  oar  or  other  stick  half  im- 
mersed in  water  seems  bent  at  the  water's  surface,  while  rivers  and 
ponds  whose  bottoms  are  visible  are  generally  deeper  than  they 
**an  to  be.  (Fig.  207.) 


§  443  REFRACTION    OF  LlOttT.  349 

Experiment  223.  — Place  a  bright  spoon  in  a  tumbler  of  water 
held  at  the  level  of  the  eye.  Bring  the  bowl  of  the  spoon  to  the 
side  of  the  glass  nearest  you.  Notice  the  appearance  of  the  spoon. 
Move  the  spoon  from  you  to  the  opposite  side  of  the  water  in  the 
tumbler.  Notice  the  changed  appearance  of  the  spoon. 

442.  Refraction. — As  a  general  thing,  when  a  lumi- 
nous beam  falls  upon  a  substance,  some  of  the  rays  are 
turned  back  or  reflected.    Other  rays  enter  the  substance, 
being  rapidly  absorbed  when  the  substance  is  opaque  or 
freely  transmitted  when  the  substance    is  transparent 
\Vo  have  now  to  consider  those  rays  that  enter  a  trans- 
parent substance.     Under  some  circumstances,  such  rays 
are  bent. 

Tliis  bending  of  a  luminous  ray  when  it  passes 
from  one  medium  to  another  is  called,  refraction 
of  light. 

443.  Refraction  Explained.— Let  us  consider  the 
passage  of  a  ray  of  light  through  a  glass  prism,  ABC. 

The  velocity  of  light  is  less 
in  glass  than  in  air  and  ihc 
direction  in  it-Inch  a  ini re- 
moves is  perpendicular  to  the 
wave  front. 

A  wave  approaches  the  side  of 

the  prism,  A  B.  When  at  a,  the  lower  end  of  the  wave 
front  first  strikes  the  glass  and  enters  it  This  end  of 
the  wave  moves  more  slowly  th:in  does  the  other,  which 
is  still  in  the  air,  and  is  continually  retarded  until  the 


350  NATURAL  PHILOSOPHY.  §  443 

whole  wave  has  entered  the  glass.     The  wave  front  thus 
assumes  the  position  shown  at  c. 

The  path  of  the  wave  being  perpendicular  to 
the  front  of  the  wave,  this  change  of  front  causes 
a  change  in  the  direction  of  the  ray  which  is 
thus  refracted  toward  a  perpendicular  to  the 
side,  AB. 

The  wave  now  moves  forward  in  a  straight  line  until 
the  top  of  the  wave  front  strikes  AC,  the  surface  of  the 
prism,  as  shown  at  m.  The  upper  end  of  the  wave  front 
emerging  first  into  the  air  gains  upon  the  other  end  of 
the  front,  which  is  still  moving  more  slowly  in  the  glass. 
When  the  lower  end  emerges  from  the  glass,  the  wave 
has  the  position  shown  at  n. 

This  second  change  of  front  involves  another 
change  in  the  direction  of  the  ray  which  is  now 
refracted  from  the  perpendicular. 

(a.)  Imagine  a  military  company  to  be  marching  in  "  column  of 
platoons,"  about  20  men  abreast.  Imagine  them  to  be  obliged  to 
march  in  a  direction  perpendicular  to  the  platoon  front.  The  line 
of  march  lies  through  a  triangular  morass  (ABC  of  Fig.  208).  At 
a,  the  soldiers  on  the  right  of  the  first  platoon  enter  the  morass, 
and  find  that  they  cannot  move  as  rapidly  as  they  had  previously 
done.  The  soldiers  on  the  left  of  the  platoon,  maintaining  their 
previous  length  and  time  of  step,  gain  on  the  right  of  the  platoon 
until  they  too  enter  the  morass.  But  this  gain  has  changed  the 
alignment  of  the  platoon  to  the  i>osition  represented  at  c.  Conse- 
quently, the  line  of  march  was  bent  or  refracted  at  the  point  where 
the  platoon  passed  from  hard  to  soft  ground.  The  velocity  being 
lessened,  the  line  was  refracted  toward  tlie  perpendicular. 


§  444  JtEFRACTIOy    OP  LtGHT.  351 

In  similar  manner,  at  m,  the  left  of  the  platoon  first  emerges 
from  the  morass  and  again  gains  upon  the  right  flank.  This 
changes  the  platoon  front  to  the  position  represented  at  n  and  re- 
fracts the  line  of  advance  from  the  perpendicular  at  the  point 
where  the  platoon  emerges  from  the  morass  and  is  able  to  increase- 
its  velocity. 

444.  Laws  of  Refraction  of  Light.— (1.)  When 
light  passes  perpendicularly  froin  one  medium 
to  another  it  is  not  refracted. 

(2.)  When  light  passes  obliquely  from  a  rarer 
to  a  denser  medium  it  is  refracted  toward  the 
perpendicular,  or  toward  a  line  drawn,  at  the  point  of 
incidence,  perpendicular  to  the  refracting  surface. 

(3.)  Wlien  light  passes  obliquely  from  a  denser 
to  a  rarer  medium,  it  is  refracted  from  the 
perpendicular. 

Experiment  224.— Place  the  bottle  shown  in  Fig.  206  upon 
several  books  resting  upon  a  table  and  invert  the  card  so  that  a 
beam  of  light  reflected  obliquely  upward  from  a  mirror  on  the 
table  may  enter  through  the  slit  near  the  bottom  of  the  bottle, 
taking  a  direction  through  the  water  similar  to  the  line  IA  of 
Fig.  212.  Notice  that  the  sunbeam  is  turned  downward  at  the 
upper  surface  of  the  water. 

Experiment  225. — Look  into  an  aquarium  in  a  direction  similar 
to  that  represented  by  the  line  IA  of  Fig.  212,  and  you  may  often 
see  images  of  the  fish  or  turtles  near  the  surface  of  the  water. 

Experiment  226.— Place  a  strip  of  printed  paper  in  a  test  tube ; 
hold  it  obliquely  in  a  tumbler  of  water  and  look  downward  at  the 
printing,  which  will  be  plainly  visible.  Change  the  tube  gradually 
toward  a  vertical  position,  and  soon  the  part  of  the  tube  in  the 
water  takes  a  silvered  appearance  and  the  printing  becomes 
invisible.  The  disappearance  of  the  reading  is  due  to  "total  re 


NATURAL  PHILOSOPHY. 


§445 


flection."  By  dissolving  a  small  bit  of  potassium  dichromate  in 
the  water,  the  tube  will  have  a  golden  instead  of  a  silver-like  ap- 
pearance. Fill  the  test  tube  with  water  and  notice  that  the  read- 
ing is  visible,  the  total  reflection  at  the  surface  of  the  air  in  the 
tube  being  destroyed. 

Experiment  227. — Place  a  bright  spoon  in  a  tumbler  of  water 
with  the  handle  leaning  from  you.  Hold 
the  tumbler  considerably  above  the  level  of 
the  eye.  Notice  that  you  see  not  only  the 
lower  part  of  the  spoon  in  the  water  but 
also  an  image  of  the  shank  of  the  spoon 
above  the  upper  surface  of  the  water  as 
represented  in  Fig.  209.  The  free  liquid 
surface  glistens  and  reflects  as  does  a 
mirror. 

445.  Total    Reflection.— When 

a  ray  of  light  attempts  to  pass  from 
a  denser  into  a  rarer  medium,  there 
are  conditions  under  which  the  angle 
of  refraction  cannot  be  greater  than 
FIG  209.  the  angle  of  incidence. 

Under  such  circumstances,  the  ray  cannot  pas* 
out  from,  the  denser  medium  but  will  be  wholly 
reflected  at  the  point  of  incidence. 

Fig.  210  represents  luminous 
rays  emitted  from  A,  under  water, 
and  seeking  a  passage  into  air. 
Passing  from  the  perpendicular, 
the  angle  of  refraction  increases 
more  rapidly  than  th'e  angle  of 
incidence  until  one  ray  is  found 


FIG.  210, 


§446 


REFRACTION  Of  LIGHT. 


353 


that  emerges  and  grazes  the  surface  of  the  water.     Rays 
beyond  this  eaunot  emerge  at  all. 

(a.)  Fig.  211  represents  a  glass  vessel  partly  filled  with  water. 
Mirrors  are  placed  at  m  and  n.     In  this 
way,  a  ray  may  be  reflected  at  m,  n  and 
o,  and  refracted  at  i. 


446.  The   Critical   Angle.— 

Imagine  a  spherical  flask  (Fig,  212) 

half  filled  with  water.     A  ray  of 

light  from  L  will  be  refracted  at  A  FIG.  ail. 

in  the  direction  of  R.    If  the  angle 

of  incidence,  CAL,  be  gradually  increased,  the  angle  of 

refraction  will  be  gradually  increased  until  it  becomes  90°, 

when  the  ray  will  graze  the  surface  of  the  water,  AM, 

If  the  source  of  light  be  still  further  removed  from  C, 

as  to  I,  the  ray  will  be  reflected  to  r. 

For  cM  m-edia,  there  is  an  incident  angle  of 
this  kind,  called  the  crit- 
ical or  limiting  angle,  be- 
yond which  total  internal 
reflection  will  take  the 
place  of  refraction. 


(a.)  The  reflection  is  called 
"total"  because  all  of  the  inci 
dent  light  is  reflected,  which  is 
never  the  case  in  ordinary  reflec- 
tion. Hence,  a  surface  at  which 
total  reflection  takes  place  con- 
stitutes the  most  perfect  mirror  possible. 


PIG.  212. 


354  NATURAL  PHILOSOPHY.  §  447 

447.  Three    Kinds  of  Refractors.— When   a  ray 

of  light  passes  through  a  refracting  medium,  three  cases 
may  arise : 

(1.)  When  the  refractor  is  bounded  by  planes,  the  re- 
fracting surfaces  being  parallel.  The  refractor  is  then 
called  a  plate. 

(2.)  When  the  refractor  is  bounded  by  planes,  the  re- 
fracting surfaces  being  not  parallel.  The  refractor  is 
then  called  a  prism. 

(3.)  When  the  refrac- 
tor is  bounded  by  two 
surfaces  of  which  at 
least  one  is  curved. 
The  refractor  is 
then  called  a  lens. 

448.    Plates.— 

^^  When  a  ray  passes 
through  a  plate,  the 
refractions  at  the  two  surfaces  are  equal  and  contrary  in 
direction.  The  direction  of  the  ray  after  passing  through 
the  plate  is  parallel  to  its  direction  before  entering. 
Objoctsseen  obliquely  through  such  plates  appear  slightly 
displaced  from  their  true  position.  An  object  at  8  would 
appear  to  be  at  S'  (Fig.  213). 

449.  Prisms. — A  prism  produces  two  simultaneous 
effects  upon  light  passing  through  it ;  a  change  of  di- 
rection and  decomposition.  The  second  of  thes.  effects 
will  be  considered  under  the  head  of  dispersion  (§  4G3). 


§450 


ItEFRACTTON  Of  LTGffiT. 


355 


(a.)  Let  mno  represent  the  section  of  a  prism.     A  ray  of  ligh*. 
from  L  being  refracted  at  a  and  6  enters  the  eye  in  the  direction 


FIG.  214 

be.     The  object  being  seen  in  the  direction  of  the  ray  as  it  enters 
the  eye  (§  433),  appears  to  be  at  r. 

(6.)  An  object  seen  through  a  prism  seems  to  be  moved  in  the 
direction  of  the  edge  that  separates  the  refracting  surfaces. 

(c,)  The  refracted  rays  are  bent  toward  the  side  that  separates 
the  refracting  surfaces,  or  toward  the  thickest  part  of  the  prism. 

450.  Lenses. — The  curved  surfaces  of  lenses  are 
generally  spherical.  With  respect  to  their  shape,  lenses 
are  of  six  kinds. 


FIG.  215. 


356  NATURAL  PHILOSOPHY.  §  450 

(1.)  Double-convex,  "1 

(2.)  Plano-convex,  '  Thl<*er  at  the  middle  tha" 


;"'  '  at  the  edges, 

(d.)  Concavo-convex,  or  meniscus,  J 

The  double-convex  may  be  taken  as  the  type  of  these. 
(4.)  Double  concave,  | 

(5.)  Plano-concave,  I  Thinner  at  the  middle  than 

(6.)  Convex-concave,  or  diverging  |      at  the  edges, 
meniscus, 

The  double -con  cave  may  be  taken  as  the  type  of  these. 

(a.)  The  effect  of  convex  lenses  may  be  considered  as  produced 
by  two  prisms  with  their  bases  in  contact ;  that  of  concave  lenses, 
by  two  prisms  with  their  edges  in  contact. 

(6.)  Tlie  effects  of  lenses  may  be  illustrated  with  spectacles  or 
eye-glasses.  The  common  magnifying:  glasses  used  in  botanical 
study  and  for  other  purposes  will  answer.  The  larger  lenses 
which  may  easily  be  removed  from  an  opera-glass  or  a  magic  lan- 
tern mny  be  made  to  furnish  the  apparatus  needed  for  our  present 
purposes.  See  description  of  "The  Water  Lens"  and  "The 
Fountain  of  Fire,"  in  the  little  book  of  "  Light  "  mentioned  at  the 
top  of  page  336. 

451.  Centre  of  Curvature ;  Principal  Axis ; 
Optical  Centre. — A  double  convex  lens,  ab,  (Fig.  216), 
may  be  described  as  the  part  common  to  two  spheres  whicl; 


PIG.  216. 


§  452  REFRACTION    OF  LIGHT.  357 

intersect  each  other.  The  centres  of  these  spheres,  c  and 
C,  are  the  centres  of  curvature  of  the  lens.  The  straight 
line,  XY,  passing  through  the  centres  of  curvature  is 
the  principal  axis  of  the  lens.  In  every  lens  there  is 
a  point,  o,  on  the  principal  axis  called  the  optical  centre. 
It  is  generally  at  equal  distances  from  the  two  faces  of 
the  lens.  Any  straight  line,  other  than  the  principal 
axis,  passing  through  the  optical  centre  is  a  secondary 
axis,  as  II K. 

Experiment  228.— Hold  one  of  the  large  lenses  of  an  opera 
glass  in  the  sun's  rays.  Notice  the  converging  pencil  formed 
by  the  rays  (after  passing  through  the  lens)  as  they  pass  through 
air  made  dusty  by  striking  together  two  blackboard  erasers.  T1i'> 
focus  and  its  distance  from  the  lens  maybe  seen.  Measure  ti.Y 
distance.  Hold  a  similar  lens  by  the  other,  face  to  face.  Noli  j 
that  the  rays  after  passing  through  both  lenses  converge  r..- . 
quickly,  lessening  the  distance  of  the  focus  from  the  lens. 

452.  Principal  Focus.—-///  incident  rays  pnr«' 
li'l  to  tJte  principal  axis  of  a  convex  lens  if/-1 
(tj'trr  tiro  refractions,  converge  at  a  point  P-<('!I  // 
the  principal  focus. 

This  point  may  lie  on  either  side  of  the  lens,  ncconl 
ing  to  the  direction  in  which  light  moves  ;  it  is  a  rc;;l 
focus.  Its  distance  from  the  lens  is  called  thefwal  dis- 
tance- 


FIG.  217. 


358 


NATURAL   PHILOSOPHY. 


§452 


(a.)  The  position  of  the  principal  focus  of  a  lens  is  easily  deter- 
mined. Hold  the  lens  facing  the  sun.  The  parallel  solar  rays  in. 
cident  upon  the  lens  will  converge  at  the  principal  focus,  F  (Fig. 
217).  Find  this  point  by  moving  a  sheet  of  paper  back  and  forth 
behind  the  lens  until  the  sunny  spot  formed  upon  the  paper  is 
as  bright  and  small  as  you  can  make  it.  Owing  to  the  identity 
between  heat  rays  and  luminous  rays,  a  convex  lens  is  also  a 
"  burning  glass." 

(6.)  Kays  diverging  from/,  a  point  at  twice  the  principal  focal 
distance  from  the  lens,  will  converge  at  f  at  twice  the  focal  dis- 
tance on  the  other  side  of  the  lens.  This  may  be  shown  by  expert 
menting  with  a  lens  and  candle-flame  until  the  flame  and  its  image 
upon  a  movable  screen  are  at  equal  distances  from  the  lens. 

(c.)  In  such  experiments,  it  is  well  to  fit  neatly  the  lens  into  a 
pasteboard  or  other  opaque  screen,  to  cut  off  from  the  screen  all 
luminous  rays  other  than  those  passing  through  the  lens. 

(d.)  The  foci  situated  at  twice  the  principal  focal  distance  are 
called  secondary  foci.  They  are  conjugate  foci. 

Experiment  229. — Hold  a  magnifying  glass  so  as  to  see  an  ob- 
ject distinctly.  Now  move  the  object  from  the  lens.  The  eye 
must  be  placed  closer  to  the  lens  to  secure  distinct  vision. 

453.  Conjugate  Foci  of  Convex  Lenses. — Kays 


.    21$. 


§453 


REFRACTION  OF  LIGHT. 


359 


diverging  from  a  luminous  point  in  the  principal  axis  at 
a  small  distance  beyond  the  principal  focus  on  either 
side  of  the  lens  will  form  a  focus  on  the  principal  axis 
beyond  the  other  principal  focus.  Thus,  rays  from  L 
will  converge  at  I  (Fig.  218)  ;  conversely,  rays  from  I  will 
converge  at  L.  The  luminous  point  and  the  focus  of 
its  rays  will  lie  in  the  same  primary  or  secondary  axis. 

Two  points  thus  related  to  each  other  are  called 
conjugate  foci ;  the  line  joining  them  always 
passes  through,  the  optical  centre. 

(a.)  When  the  luminous  point  is  at  the  focal  distance,  the  re- 
fracted rays  will  be  parallel  (Fig.  219  [;] )  and  no  focus  will  be 
formed. 


FIG.  219. 

(6.)  When  the  luminous  point,  L,  is  at  less  than  the  focal  dis- 
tance, the  refracted  rays  will  still  diverge  as  if  from  a  point,  I,  on  the 
same  side  of  the  lens,  more  distant  than  the  principal  focus.  (Fig. 
219  [2 ]  ).  This  focus  will  be  virtual. 

(c.)  Conversely,  converging  rays  falling  upon  a  convex  lens  will 
form  a  focus  nearer  the  lens  than  the  principal  focus. 


360  NATURAL    PHILOSOPHY.  §  45/1. 

454.  Images    Formed    by    Convex    Lenses.— 
The  analogies  between  the  convex  lens  and  the  concave 
mirror  cannot  have  escaped  the  notice  of  the  thoughtful 
pupil.     Others  will  appear. 

If  secondary  axes  be  nearly  parallel  to  the  principal 
axis,  well-defined  foci  may  be  formed  upon  them,  as  well 
as  upon  the  principal  axis.  A  number  of  these  foci 
may  determine  the  position  of  an  image  formed  by  a 
lens. 

455.  Diminished    Real    Image.—//  the  Abject. 
A  B,  be  more  than  twice  the  focal  distance  from 
the  convex  lens,  its  image  will  be  real,  smaller 
than  the  object  and  inverted.     (Fig.  220.) 


FIG.  220. 


Kays  proceeding  from  A  are  focused  at  a  upon  the 
secondary  axis,  A0a\  similarly  with  rays  from  B  and 
from  all  intermediate  points.  Hence  the  image,  ab. 

456.  Magnified  Real  Image.--//  the  object  be 
further  from  the  lensfwn  the  principal  focus,  but 


§457 


REFRACTION  OF  LIGHT. 


361 


at  a  distance  less  than  twice  the  focal  distance, 
the  image  will  be  real,  magnified  and  inverted. 
(Fig.  221.) 


FIG.  221. 

457.  Virtual  Image. — If  the  object,  AB,  be  placed 
nearer   the    lens   than   the    principal   focus,    the 


FIG.  232. 

image  will  be  virtual,  magnified  and  erect. 
from  .1  appear  to  come  from  a;  rays  from  B  appear  to 
come  from  b. 

Experiment  230. — Repeat  Experiment  228,  using  the  eyt* 
glasses  or  small  leases  of  an  opera  glass.  Notice  th.at  the  re- 
fracted rays  form  a  diverging  pencil. 


362  NATURAL  PHILOSOPHY.  §  458 

458.  Conjugate  Foci  of  Concave  Lens.— Rays 
from  a  luminous  point  at  any  distance  whatever  will 
be  made  more  divergent  by  passing  through  a  concave 


lens.  Rays  parallel  to  the  principal  axis  will  diverge 
after  refraction  as  if  they  proceeded  from  the  principal 
focus.  In  any  case,  the  focus  will  be  virtual  and  nearer 
the  lens  than  the  luminous  point.  In  Fig.  223,  the  vir- 
tual image  of  L  is  at  I,  from  which  point  the  refracted 
rays  appear  to  proceed. 

459.  Image  formed  by  Concave  Lenses.— 
Images  formed  by  a  concave  lens  are  virtual, 
tmaller  than  the  object  and  erect. 


§460 


RECA  P  ITU  LA  TION. 


363 


460.  Recapitulation. — To  be  amplified  by  tbe  pupil 
for  veview. 

REFRACTION  OF  LIGHT. 


0 

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P* 

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F  !  1 

j               a 

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PJ  ^j^_ 

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3                   0,       ^3 

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^                                   ^                M 

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564  NATURAL  PHILOSOPHY.  §  460 


EXERCISES. 

1.  (a.)  What  is  refraction  of  light?  (6.)    State  the  laws  govern- 
ing the  same,     (c.)  Give  an  illustrative  diagram. 

2.  (a.)  Name  and  illustrate  by  diagram  the  different  classes  of 
lenses.     (6.)  Explain,  with  diagram,  the  action  of  the  burning- 
glass. 

3.  Explain  total  reflection. 

4.  Show,  with  diagram,  how  the  secondary  axes  of  a  lens  mark 
the  limits  of  the  image. 

5.  Using  a  convex  lens,  what  must  be  the  position  of  an  object 
in  order  that  its  image  shall  be  real,  magnified  and  inverted  ? 

6.  Show,  by  a  diagram,  the  course  of  a  ray  of  light  passing 
through  the  centre  of  a  glass  sphere. 

7.  Show,  by  a  diagram,  the  course  of  a  ray  of  light  passing 
through  a  glass  sphere  at  one  side  of  the  centre. 

8.  Draw  the  section  of  a  prism  and  draw  lines  showing  the  path 
of  a  ray  of  light  through  it. 

9.  A  cathetal  prism  (one  whose  section  is  an  isosceles,  right- 
angled  triangle)  may  be  used  as  a  mirror.     Draw  a  diagram  show- 
ing how  this  may  be  done. 

10.  Draw  diagrams  showing  the  direction  of  rays  of  light  before 
and  after  refraction  by  a  double  convex  lens,  the  rays  starting  from 
a  luminous  point  (a.)  at  the  principal  focus.     (6.)  On  the  principal 
axis  between  the  principal  and  secondary  foci,     (c.)  On  the  prin- 
cipal axis  beyond  the  secondary  focus,     (d.)  On  the  principal  axis, 
lietween  the  lens  and  the  principal  focus. 


SECTION    IV. 

CHROMATICS.— SPECTRA. 

461.  Other  Results  of  Refraction.— Most  lumi- 
nous objects  emit  light  of  several  kinds  blended  together. 
We  must  learn  to  sift  these  varieties  one  from  the  other 
and  to  deal  with  any  one  kind  by  itself. 


F'TG.  224. 

Experiment  231. — Admit  a  sunbeam  through  a  very  small 
opening  in  the  shutter  of  a  darkened  room  The  opening  may  be 
prepared  by  cutting  a  slit  an  inch  (25  mm.)  long  and  •£%  of  an 
inch  (1  mm.}  wide  in  a  card.  See  that  the  edges  of  the  slit  are 
smooth.  Tack  the  slit  over  a  larger  opening  in  the  shutter.  If 
we  look  at  the  aperture  from  E  (Fig.  224),  we  shall  see  the  pun 
beyond.  The  path  of  the  beam  from  S  to  E  is  made  visible  by  the 
floating  dust. 

If  a  prism  be  placed  in  the  path  of  the  beam ,  as  shown  in  the 
figure,  the  sides  of  the  slit  and  edges  of  the  prism  Wing  horizontal, 
the  beam  will  be  refracted  upward.  If  the  refracted  beam  be 


366  NATURAL  PHILOSOPHY.  §  462 

caught  upon  a  screen,  it  will  appear  as  a  band  of  differently 
colored  light,  passing  very  gradually  from  red  at  the  bottom, 
through  orange,  yellow,  green,  blue  and  indigo  to  violet  at  the 
upper  end  of  the  beautifully  colored  band.  By  placing  the  slit  in 
a  vertical  position  and  standing  the  prism  on  its  end  so  that  its 
edges  will  be  parallel  with  the  sides  of  the  slit,  the  spectrum  m:iy 
be  projected  as  a  horizontal  band. 

The  rays  thus  separated  by  the  prism  are  identical  with  each 
other  and  with  radiant  heat  except  in  the  matter  of  wave  length 
or  (what  amounts  to  the  same  thing)  rapidity  of  vibration. 


462.  The    Solar    Spectrum.— The  colored  land 
produced   in  Experiment  231  is  called  the  solar 
-pectruin. 

The  initials  of  the  names  of  these  different  colors  f,  r - 
the  meaningless  word,  VIBGYOR,  which  may  aid  the  ]• 
pil  in  remembering  these  prismatic  colors  in  their  pro  ,  r 
order. 

463.  Dispersion.— By  looking  at  Fig.  224,  it  will  l.o 
seen  that  the  red  rays  have  been  refracted  the  least  and 
the  violet  the  most  of  all  the  luminous  rays.     Tfiis  sejxt- 
ration  of  the  differently  colored  rays  by  the  prism 
is  called  the  dispersion  of  light ;  it  depends  upon  the 
fact  that  rays  of  different  colors  arc  refracted  in  different 
degrees. 

Experiment  232. — Let  the  rays  that  have  been  dispersed  by  a 
prism  fall  upon  a  convex  lens  as  shown  in  Fig.  225.  They  will 
be  refracted  to  a  focus  and  recombined  to  form  white  light.  A 
toncave  mirror  may  be  used  to  reflect  the  rays  to  a  focus  instead 
of  using  the  lens  as  above  described. 


§463 


CUE  OMA  TICS— SPECTRA . 


367 


PIG.  225. 

Experiment  233.— Make  a  "Newton's  disc"  of  cardboard 
painted  with  the  prismatic  colors  in  proper  proportion  as  indicated 
by  Fig.  226.  It  is  better  to  divide  the  surface  given  to  each  color 
into  smaller  sectors  arranged  alternately  as  shown  in  Fig.  227.  You 


FIG.  226. 


FIG.  227. 


may  paste  sectors  of  properly  colored  paper  upon  the  card-board 
instead  of  painting  them.  Cause  this  disc  to  revolve  rapidly  by 
means  of  the  whirling  table  or  by  fastening  it  to  a  large  top. 
Notice  that  the  colors  are  blended  and  that  the  disc  appears 
grayish  white. 

Experiment  234.— Hold  a  second  prism  near  one  that  is  used  to 
produce  a  solar  spectrum,  the  position  of  the  second  being  inverted 
with  reference  to  the  first.  If  the  dispersing  prism  be  held  as 
shown  in  Fig.  224  or  225,  the  second  should  be  held  with  the 
refracting  edge  uppermost,  the  facing  surfaces  being  parallel. 


36$  NATURAL  PfflLdSdP&f.  §  464 

The  dispersed  rays  emerging  from  the  first  prism  will  pass 
through  the  second.  The  rays  separated  by  the  first  will  be 
again  blended  by  the  second  and  appear  as  white  light. 

Experiment  235. — Hold  a  hand  mirror  near  the  dispersing  prism 
so  as  to  reflect  the  refracted  rays  to  a  distant  wall  or  ceiling.  Give 
to  the  mirror  a  rapid,  angular  motion  so  that  the  spectrum  is  made 
to  move  to  and  fro  very  quickly  in  the  direction  of  its  length. 
The  spectrum  changes  to  a  band  of  white  light  with  a  colored 
spot  at  each  end.  The  effect  is  due  to  what  is  known  as  the 
"Persistence  of  Vision,"  familiarly  illustrated  by  the  experi- 
ment of  producing  a  ring  of  light  by  whirling  a  firebrand  around 
a  circle. 

464.  The  Composition  of  White  Light.— We 
have  now  shown,  by  both  the  processes  of  analysis  and 
synthesis,  that  white  light  is  composed  of  the  seven 
prismatic  colors.  We  have  decomposed  white  light 
into  its  seven  constituents  and  recombined  these  constit- 
uents into  white  light. 

Experiment  236. — Paint  three  narrow  strips  of  cardboard,  one 
vermilion  red,  one  emerald  green,  and  the  other  aniline  violet. 
Be  sure  that  the  coats  are  thick  enough  thoroughly  to  hide  the 
cardboard.  When  dry,  hold  the  red  strip  in  the  red  of  the  solar 
spectrum  ;  it  appears  red.  Move  it  slowly  through  the  orange  and 
yellow  ;  it  grows  gradually  darker.  In  the  green  and  colors  be- 
yond, it  appears  black.  Repeat  the  experiment  with  the  other 
two  strips  and  carefully  notice  the  effects. 

Experiment  237.— Make  a  loosely  wound  ball  of  candle  wick  ; 
soak  it  in  a  strong  solution  of  common  salt  in  water  ;  squeeze  most 
of  the  brine  out  of  the  ball  ;  place  the  ball  in  a  plate  and  pour  al- 
cohol over  it.  Take  it  into  a  dark  room  and  ignite  it.  Examine 
objects  of  different  colors,  as  strips  of  ribbon  or  cloth,  by  this  yellow 
light.  Only  yellow  objects  will  have  their  usual  appearance 


§  4^7  CHROMATICS—  SPECTRA.  $6$ 

465.  Color  of  Bodies.— The  color  of  a  body  is  its 
property  of  reflecting  or  transmitting  to  the  eye  rays  of 
&  particular  kind,  the  other  rays  being  generally  absorbed. 

(a.)  Properly  speaking,  color  is  not  a  property  of  matter,  but  of 
.iglit.  A  ribbon  is  called  red,  but  the  redness  belongs  to  the  light, 
not  to  the  ribbon.  There  would  be  more  propriety  in  saying  that  the 
ribbon  has  ail  the  other  colors  of  the  rainbow,  because  it  keeps  the 
others  and  reflects  the  red.  If  the  red  ribbon  be  placed  in  the  green 
or  blue  of  the  spectrum,  it  will  appear  black  because  it  receives  no 
red  rays  to  reflect. 

466.  The    Rainbow. — The   rainbow  is  due  to  re- 
fraction, reflection  and  dispersion  of  sunlight  by  water- 
drops.    The  necessary  conditions  are  : 

(1.)  A  shower  during  sunshine. 

(2.)  That  the  observer  shall  stand  with  his  back  to 
the  sun,  between  the  falling  drops  and  the  sun.  (See 
Elements  of  Natural  Philosophy,  §§  706-710.) 

467.  The    Luminous   Spectrum. — The  color  of 
light  depends  only  upon  wave  length  or  rapidity 
of  vibration.     Color  is  to  optics  what  pitch  is  to  acous- 
tics. 

"Red  light  has  a  wave  length  o.f  STJoiF  nicn  5  violet  light 
lias  a  wave  length  of  -g-r&w  inch.  The  wave  lengths 
that  correspond  to  the  other  colors  are  intermediate  be- 
tween these  in  value. 

The  shorter  the  wave,  the  more  it  is  refracted  (See  Fig. 
224).  Of  course,  the  wave  length  depends  upon  the 
rapidity  of  vibration  of  the  molecule  that  produced  the 
wave  in  the  ether. 


370  NATURAL  PHILOSOPHY.  §  468 

468.  Other  Properties  of  the   Sunbeam.— We 
have   decomposed  a  sunbeam,  and  thus  produced  the 
seven  prismatic  colors.     But  we  must  go  still  further. 
Beyond  the  limits  of  the  visible  spectrum,  in,  both 
directions,  there  are  rays  that  do  not  excite  the  op- 
tic nerve,  the  existence  of  which,  however,  may  be  easily 
proved.     These  visible  and  invisible  rays  differ  only  in 
respect  to  wave  length.    Some  of  these  waves  are  so  short 
and  fall  so  rapidly  that  the  optic  nerve  cannot  respond 
to  their  vibrations.     OtLers  are  so  long  and  fall  so  slowly 
that  the  optic  nerve  cannot  respond  to  their  vibrations 
either.    In  either  case,  the  rays  are  incapable  of  exciting 
vision. 

469.  Actinic    Spectrum. — The   ac'inic    or   chemi- 
cal effects  of  sunlight  are  familiar  to  all.     The  sensitive 
paper  of  the  photographer  will  remain  unchanged  in  the 
dark  ;  it  will  be  quickly  blackened  in  the  light. 

If  a  piece  of  paper,  freshly  washed  in  a  solution  of  sul- 
phate of  quinine,  be  held  successively  in  the  different 
parts  of  the  visible  spectrum,  it  will  be  affected  least  in 
the  red  and  most  in  the  violet. 

In  this  way  actinic  effects  may  be  found  at  a 
point  beyond  tJie  violet^  wholly  outside  the  visible 
spectrum.  We  thus  detect  ultra-violet  rays  con- 
stituting an  actinic  spectrum. 

A  quartz  prism  is  desirable  for  this  experiment,  as 
glass  quenches  most  of  the  actinic  rays. 

470.  Thermal   Spectrum. — If  a  very  delicate  ther- 


§471 


RECA  PITVLA  TIOX. 


371 


mometer  or  thermopile  be  successively  placed  in  various 
parts  of  the  spectrum,  it  will  be  found  that  the  tempera- 
ture  is  scarcely  affected  in  the  violet,  but  that  there  is  a 
continual  increase  in  temperature  as  the  thermometer 
is  moved  toward  the  other  end  of  the  spectrum,  it  being 
quite  marked  in  the  red. 

A  very  marked  rise  of  temperature  takes  place 
beyond  the  red,  wholly  outside  the  visible  spec- 
trum. We  thus  detect  ultra-red  rays  constitut- 
ing a  heat  spectrum. 

A  rock-salt  prism  is  desirable  for  this  experiment,  as 
glass  absorbs  most  of  the  ultra-red  rays. 

Radiant  heat  differs  from  light  only  in  wave  length. 
All  luminous  rays  have  heating  power  ;  heat  rays  are 
luminous  if  their  vibrations  come  at  such  a  rate  that  the 
organ  of  sight  can  absorb  their  energy  and  respond  with 
sympathetic  vibrations  (§  338). 

471.  Recapitulation.— To  be  amplified  by  the  pupil 
for  review. 

C  By  Prisms. 
(_  By  Water  Drops. 
f  Luminous. 
Actinic. 


COMPLEXITY   OF 
SUNBEAM.... 


COLOR  OF  BODIES. 


ANALYSIS. 


Spectra 

Thermal. 
By  Lenses. 
By  Mirrors. 
By  Prisms. 
By  Persistence  of  Vision. 


NATURAL  PHILOSOPHY.  §  471 


EXERCISES. 

1.  What  is  the  difference  between  the  "  diffusion  of  light "  and 
"  the  dispersion  of  light  "  ? 

2.  Why  does  a  red  body  appear  red  ? 

3.  Name  the  colors  of  the  solar  spectrum  in  their  proper  order, 
beginning  with  the  one  of  least  wave  length. 

4.  To  what  quality  of  sound  does  color  correspond  ? 

5.  Why  does  the  moon  look  green  when   viewed  through  a 
piece  of  green  glass  ? 

6.  You  hold  in  your  hand  a  red  rose.    You  carry  it  into  a  room 
that  is  wholly  dark.     Has  the  rose  then  any  color?    Explain  your 
answer. 

7.  Imagine  a  line  186,000  miles  long  extending  from  your  eye 
toward  the  sun.    Radiant  energy  (heat  or  light)  will  traverse  such  si 
line  in  a  second  (§  427).     Suppose  that  light  from  the  sun  is  com- 
ing toward  you.    (a.)  After  the  first  wave  reaches  the  far  end  of 
this  line,  how  long  will  it  take  the  same  wave  to  reach  you  V    (b.) 
If  the  light  is  red,  how  many  such  waves  can  lie  on  this  line  r.t 
once  ?  (c.)  How  many  such  waves  will  enter  your  eye  in  a  second  ? 

8.  You  see  a  red  ribbon,    (a.)  Does  the  ribbon  reflect  any  light  ? 
(It.)  How  do  you  know  ?    (c.)  If  it  does,  what  is  the  color  of  the  re 
fleeted  light  ?    (d.)  Some  of  the  sun's  rays  enter  the  ribbon  ;  what 
is  their  effect  ? 

9.  "  Converging  "  and  "  diverging  "  lenses  are  sometimes  men- 
tioned.    What  are  they? 

10.  You  see  a  rainbow.     State  the  threefold  work  done  upon  a 
sunbeam  by  each  raindrop  that  contributes  to  the  success  of  the 
exhibition. 


SECTION     V. 


A  FEW  OPTICAL  INSTRUMENTS. 

472.  Photographer's  Camera. —  The  photogra- 
pher's camera  is  nearly  the  same  as  the  camera-obscura 
described  in  §  426.  Instead  of  the  darkened  room  we 
have  a  darkened  box  ;  instead  of  the  simple  hole  in  the 
shutter  we  have  a  convex  lens,  placed  in  a  tube  at  A. 
(Fig.  228.) 

(a.)  A  ground-glass  plate  is  placed  in  the  frame  at  E,  which  is 
adjusted  so  that  a  well- 
defined,  inverted  image  of 
the  object  in  front  of  A  is 
projected  upon  the  glass 
plate.  This  adjustment 
or  "focusing"  is  com- 
pleted by  moving  the  lens 
and  it  3  tube  by  the  toothed 
wheel  at  D  until  the  ob- 
ject in  front  of  A  and  the 
plate  at  E  are  at  the  con- 
nigate  foci  of  the  lens  at 
A.  When  the  "  focus- 
ing" is  satisfactory,  A  is  covered  with  a  cloth,  the  ground-glass 
plate  is  replaced  by  a  chemically-prepared  sensitive  plate,  the  cloth 
removed  and  the  image  projected  on  the  plate.  The  light  works 
certain  chemical  changes  where  it  falls  upon  this  plate  and  thus  a 
more  lasting  image  is  produced.  The  many  other  processes  in- 
volved in  photography  cannot  be  considered  here. 

473-  The  Human  Eye. — This  most  admirable  of 
all  optical  instruments  is  a  nearly  spherical  ball,  capable 


FIG.  228. 


374 


NATURAL   PHILOSOPHY. 


473 


of  being  turned  considerably  in  its  socket.  The  outer 
coat,  S,  is  called  the  white  of  the  eye.  Its  transparent 
part  in  front,  C,  is  called  the  cornea.  The  cornea  is 
more  convex  than  the  rest  of  the  eyeball.  The  cornea 
fits  into  the  coat,  S,  as  a  watch  crystal  does  into  its  case. 
Behind  the  coat,  S,  is  a  dark,  opaque  coat,  N.  Behind 
the  cornea  -is  a  curtain,  /,  called  the  iris.  It  is  colored 
and  opaque ;  the  circular  window  in  its  centre  is  called 
the  pupil.  The  color  of 
the  iris  constitutes  the  color 
of  the  eye.  Back  of  the 
pupil  is  the  crystalline 
lens,  L,  built  of  concentric 
shells  (layer  on  layer,  as  in 
an  onion)  of  varying  den- 
sity. Its  shape  is  shown  in 
the  figure.  This  lens  di- 
vides the  eye  into  two 
chambers.  The  anterior 
chamber  contains  a  limpid  liquid  called  the  aqueous 
humor ;  the  posterior  chamber  contains  a  transparent 
jelly,  F.  called  the  vitreous  humor. 

The  cornea,  aqueous  humor,  crystalline  lens  and 
vitreous  humor  are  refracting  media.  Back  of  the 
membrane,  H,  which  incloses  the  vitreous  humor,  is  the 
retina,  R,  an  expansion  of  the  optic  nerve. 

The  eye,  optically  considered,  is  simply  art  ar- 
rangement for  projecting  inverted,  real  images  of 
visible  objects  upon  a  screen,  li,  made  of  nerve 
filaments.  If  the  images  thus  formed  be  well  defined 
and  sufficiently  luminous,  the  vision  is  distinct. 


FIG. 


§  475  OPTICAL  INSTRUMENTS.  375 

(a.)  Objects  are  not  seen  distinctly  unless  the  image  falls  di- 
rectly upon  the  retina.  If  the  image  of  a  distant  object  falls  in 
front  of  the  retina,  the  person  is  said  to  be  near-sighted.  Near- 
sightedness  may  be  relieved  by  the  use  of  concave  glasses. 

If  the  image  of  a  near  object  tends  to  fall  back  of  the  retina, 
the  person  is  said  to  be  far-sighted.  Far-sightedness  may  be  re- 
lieved by  the  use  of  convex  glasses. 

(b.)  The  eye  of  the  common  house-fly  is  said  to  have  4,000 
lenses ;  that  of  some  kinds  of  beetles  is  said  to  have  25,000,  each 
with  its  own  cornea  and  retina.  "  If  each  of  these  lenses  forms  a 
separate  picture  of  each  object,  what  an  awful  army  of  cruel  giants 
must  the  beetle  behold  when  he  is  captured  by  a  schoolboy  I" 

474.  Magnifying  Glasses.—  A  magnifying  glass,  or 
simple  microscope,  is  a  con- 
vex lens,    generally   double- 
convex.      The    object     is 

placed  between  the  lens  and 
its  principal  focus.  The 
image  is  virtual,  erect  and 
magnified. 

475.  Compound    Mi- 
croscope.— The  compound 
microscope  consists  of   two 
or  more  convex  lenses  placed 
iu  a  tube.     One  of  these,  o, 
called    the    object-glass    or 
objective,   is  of  short  focus. 
The  object,  ab,  being  placed 
slightly  beyond  the  principal 
focus,  a  real  image,  cd,  mag- 
nified and  inverted,  is  formed 

within  the  tube  (§  456),  Fio.  380. 


376 


NATURAL   PHILOSOPHY. 


§475 


The  other  lens,  E,  called  the  eye-glass,  is  so  placed 
that  the  image  formed  by  the  objective  lies  between  the 
eye-glass  and  its  focus.  AB  (Fig.  230)  is  a  magnified, 
virtual  image  of  the  real  image,  formed  by  the  eye-glass 
and  seen  by  the  observer. 

476.    Galilean    Telescope ;    Opera    Glass.  —  In 

the  telescope  attributed  to  Galileo,  the  objective,  0,  is 
a  double  convex  and  the  eye-piece,  C}  is  a  double  con- 
cave lens.  The  concave  lens  intercepts  the  rays  before 
they  have  reached  "the  focuc  of  the  objective. 


Fio.  231. 

The  rays  from  A,  converging  after  refraction  by  0, 
are  rendered  diverging  by  C ';  they  seem  to  diverge  from 
a.  In  like  manner,  the  image  of  B  is  formed  at  b. 
The  image,  ab,  is  erect  and  very  near.  An  opera  glass 
consists  of  two  Galilean  telescopes  placed  side  by  side. 

477.  Astronomical  Telescope  ;  Refractor. — As- 
tronomical telescopes  are  of  two  kinds — refractors  and 
reflectors.  Fig.  232  represents  the  arrangement  of  the 
lenses  and  the  direction  of  the  rays  in  the  refracting 
telescope.  The  object  glass  is  of  a  large  diameter  that 
it  may  collect  many  rays  for  the  better  illumination  qi 
the  image. 


§478 


OPTICAL  INSTRUMENTS. 


377 


FIG.  232. 

The  inverted,  real  image,  ab,  formed  by  the  objective, 
0,  is  magnified  by  the  eye-piece,  as  in  the  case  of  the 
compound  microscope.  The  visible  image,  cd,  is  a  vir- 
tual image  of  ab,  which  is  the  real  image  of  AB.  The 
lenses  are  enclosed  in  a  tube. 

(a.)  The  largest  refracting  telescope  (1884)  is  at  the  Pulkowa 
Observatory.  Its  object  glass  is  30  inches  in  diameter.  A  36 
inch  object  glass  is  making  for  the  telescope  of  the  Lick  Obser- 
vatory in  California 

478.  Reflecting  Telescopes.— A  reflecting  tele- 
scope consists  of  a  tube  closed  at  one  end  by  a  concav.e 


FIG.  233. 


mirror,  M,  so  placed  that  the  image  formed  by  it  maj 

be  magnified  by  a  convex  lens  used  as  an   eye-piece. 

The  rays  from  the  mirror  are  reflected  at  mn,  andareaJJ 


£78  NATURAL   PHILOSOPHY.  §  478 

image  formed  at  db.  This  image  is  magnified  by  the 
glasses  of  the  eye-piece  and  a  virtual  image  formed  &ted. 
The  Earl  of  Rosse  built  a  telescope  with  a  mirror  six 
feet  in  diameter  and  having  a  focal  distance  of  fifty-four 
feet. 

Experiment  238. — Reflect  a  horizontal  beam  of  sunlight  into 
a  darkened  room.  In  its  path,  place  a  piece  of  smoked  glass 
on  which  you  have  traced  the  representation  of  an  arrow,  AS, 
(Fig.  234)  or  written  your  autograph.  Be  sure  that  every  stroke  of 


Fro.  234. 

the  pencil  has  cut  through  the  lamp  black  and  exposed  the  glass 
beneath.  Place  a  convex  lens  beyond  the  pane  of  glass,  as  at  L, 
so  that  rays  that  pass  through  the  transparent  tracings  may  be 
refracted  by  it  as  shown  in  the  figure.  It  is  evident  that  an  image 
will  be  formed  at  the  foci  of  the  lens.  If  a  screen,  SS,  be  held  at 
the  positions  of  these  foci,  a  and  6,  the  image  will  appear  clearly 
cut  and  bright.  If  the  screen  be  held  nearer  the  lens  or  further 
from  it,  as  at  ff  or  8",  the  picture  will  be  blurred. 

479.  Magic  Lantern. — In  the  magic  lantern  (Fig. 
235),  a  lamp  is  placed  at  the  common  focus  of  a  convex 
lens  (called  the  "condenser")  in  front  of  it  and  of  a  con- 
cave mirror  behind  it.  The  light  is  thus  concentrated 
upon  ab,  a  transparent  picture,  called  the  "slide."  A 
of  lenses,  m,  is  placed  at  a  little  more  than  its 


§479 


OPTICAL  INSTRUMENTS. 


379 


FIG.  235. 


focal  distance  beyond  the  slide.  A  real,  inverted,  mag- 
nified image  of  the  picture  is  thus  projected  upon  the 
screen,  8.  The  tube  carrying  in  is  adjustable,  so  that 
the  foci  may  be  made  to  fall  upon  the  screen  and  thus 
render  the  image  distinct.  By  inverting  the  slide,  the 
image  is  produced  right  side  up. 

(a.)  Directions  for  making  a  simple  magic  lantern  may  be  found 
on  page  84  of  Mayer  and  Barnard's  little  book  on  Light.    Fig.  23B 


FIG.  336. 


NATURAL  PBILOSOPHT. 


/"•         1 


if 


represents  a  very  compact  and  efficient  lantern,  known  as  Marcy's 
Sciopticon  and  furnished  by  James  W.  Queen  &  Co.,  of  Philadelphia. 

Experiment  239.—  Close  the  left  eye  and  hold  the  right  hand  so 
that  the  forefinger  shall  hide  the  other  three  fingers.     Without 
changing  the  position  of  the  hand,  open  the  left  and  close  the  right 
eye.     The   hidden  fingers  become 
visible  in  part. 

Experiment  240.—  Place  a  die  on 
the  table  directly  in  front  of  you. 
Looking  at  it  with  only  the  left  eye, 
three  faces  are  visible,  as  shown  at 
A,  Fig.  337.  Looking  at  it  with 
only  the  right  eye,  it  appears  us  shown  at  B. 

480.  Stereoscopic  Effects.  —  Jrom  the  last  two  ex- 
periments, we  see  that  when  we  look  aS  a  solid,  the 
images  upon  the  retinas  of  the  two  eyeo  are  dif- 
ferent. If,  in  any  way,  we  combine  two  drawings,  so  a? 
to  produce  images  upon  the  retinas  of  the  two  eyes  like 
those  produced  by  the  solid  object,  we 
obtain  the  idea  of  solidity. 

481.    The    Stereoscope.  —  To 

blend  these  two  pictures  is  the  office 
of  the  stereoscope.  Its  action  will  be 
readily  understood  from  Fig.  238. 
The  diaphragm,  D,  prevents  either 
eye  from  seeing  both  pictures  at  the 
same  time.  Kays  of  light  from  B  are 
refracted  by  the  half-lens,  E',  so  that 
they  seem  to  come  from  C.  In 
the  same  way,  rays  from  A  are  re- 
Fio.  m  fracted  by  E  so  that  they  also  seem 


o  A.o2  JlECA FiTULA  TlOffl*  381 

to  come  from  C.  The  two  slightly  different  pictures, 
thus  seeming  to  be  in  the  same  place  at  the  same  time, 
are  successfully  blended  and  the  picture  "stands  out" 
or  has  the  appearance  of  solidity.  If  the  two  pictures 
of  a  stereoscopic  view  were  exactly  alike,  this  impression 
of  solidity  would  not  be  produced. 

482.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 

OBSCURA. 
CAMERA... 


.      HUMAN  EYE. 
£ 


(  OBSCURA. 
PHOTOGRAPHER'S. 

I 


SIMPLE. 
MICROSCOPES. 

COMPOUND. 


{  REFLECTORS. 
REFRACTORS . 


MAGIC   LANTERN. 


STEREOSCOPE. 


Galilean. 
Opera  Glats. 
Astronomical. 


CONCLUSION. 

ENERGY. 

483.  Varieties  of  Energy. — Like  matter,  energy 
is  indestructible.    We  have  already  seen  that  energy  may 
be  visible  or  invisible  (i.  e.,  mechanical  or  molecular), 
kinetic  or  potential.     We  have  at  our  control  at  least 
eight  varieties  of  energy. 

(a.)  Mechanical  energy  of  position  (visible,  potential). 
(6.)  Mechanical  energy  of  motion  (visible,  kinetic). 
(e.)  Latent  heat  (molecular,  potential). 
(d.)  Sensible  heat  (molecular,  kinetic). 
(e. )  Chemical  separation  (molecular  or  atomic  ;  potential). 
(/.)  Electric  charges  (probably  molecular,  potential). 
(g. )  Electric  currents  (probably  molecular,  kinetic). 
(h.)  Radiant  energy,  thermal,  luminous  or  actinic  (molecuiar, 
kinetic). 

484.  Conservation  of  Energy. — The  doctrine  that, 
considering  the  universe  as  a  whole,  the  sum  of  all  the 
forms  of  energy  is  a  constant  quantity,  is  known  as  the 
Conservation  of  Energy. 

a+b+c+d+e  +f+g  +  h=&  constant  quantity. 

This  does  not  mean  that  the  value  of  a  is  invariable  ; 
we  have  seen  it  changed  to  other  varieties  as  b  or  d.  We 
have  seen  heat  changed  to  electricity  and  vice  versa,  and 
either  or  both  changed  to  mechanical  energy.  It  does 
not  mean  that  the  sum  of  these  eight  variable  quantities 
in  the  earth  is  constant,  for  we  have  seen  that  energy 


§486 


ENERGY. 


383 


may  pass  from  sun  to  earth,  from  star  to  star.  But  it 
does  mean  that  the  sum  of  all  these  energies  in 
all  the  worlds  that  constitute  Uie  universe  is  a 
quantity  fixed,  invariable. 

485.  Correlation  of  Energy. — The  expression  Cor- 
relation  of  Energy  refers  to  the  convertibility  of  one 
form  of  energy  into  another.     Our  ideas  ought,  by  this 
time,  to  be  clear  in  regard  to  this  convertibility.     One 
important    feature    remains  to  be  noticed.     Radiant 
energy  can  be  converted  into  other  forms,  or  other 
forms  into  radiant  energy  only  through  the  in- 
termediate state  of  absorbed  heat. 

486.  Recapitulation. — To  be  amplified  by  the  pupil 
for  review. 


VISIBLE  OR  MECHANICAL. 


OfPotition 
Potential. 


Of  Motion 
Kinetic. 


e.g.,  Hanging     Apple, 
Head  of  Water. 

e.g.,  Falling     Apple, 
Flowing  Water. 


INVISIBLE  OR 
MOLECULAR. 


{Of  Position,  e.  g..  Latent  Heat. 
Potential. 
Of  Motion,  e.g..  Sensible  Heat. 
Kinetic: 

Of  Motion, 


ELECTRICITY  . 


Of  Position,  e.  g.,  Charged      Leyden 
Potential.       jar,   Battery  with  cir- 
cuit broken. 

Of  Motion,  e.  g.,    Leyden    jar    dis- 
Kinttic.       charging  ;  Battery  with 
circuit  closed. 


GENERAL 

1.  (a.)  Define  science,  matter,  mass,  molecule  and  atom.     (6.) 
Define  physics. 

2.  (a.)  What  are  physical  properties  of  matter?     (6.)  Define 
and  illustrate  two  properties  of  matter. 

3.  Define  solid,  liquid  and  gas  ? 

4.  (a.)  Give  Newton's  laws  of  motion.    (6.)  Give  the  law  of  re- 
flected motion. 

5.  How  may  we  measure  distances  by  sound  ? 

6.  Upon  what  does  the  pitch  of  a  sound  depend  ? 

7.  Why  do  the  rays  of  the  evening  sun  come  to  us  in  curved 
lines  ? 

8.  How  does  distance  effect  the  intensity  of  light  ? 

9.  What  is  an  angle  of  reflection  ? 

10.  (a.)  What  is  the  general  effect  of  a  concave  mirror  ?    (5.)  Of 
a  concave  lens  ? 

11.  Fig.   339  represents  reflection  of  luminous  rays  from  a 

rough  surface.  Draw  a 
similar  figure  illustrat- 
ing reflection  by  a  plane 
mirror. 

12.  Is  light  always  re- 
fracted   in    passing  from 
one  medium  to  another  ? 

13.  How  does  a  straight 
stick  appear  when  partly 
immersed     in    water ? 
Why? 

14.  Under    what    cir- 
cumstances  will  the  object 

and  the  image  be  on  the  same  side  of  a  convex  lens  ? 

15.  Why  will  an  iron  gate  that  opens  easily  in  winter  often 
ttick  in  summer  ? 


§  486  GENERAL  REVIEW.  385 

16.  What  is  meant  by  intermolecular  spaces  and  how  do  they 
compare  with  molecular  diameters? 

17.  How  are  centigrade  and  Fahrenheit  thermometers  graded  ? 

18.  Why  are  inland  cities  subject  to  greater  extremes  of  tem- 
perature than  seaside  resorts  ? 

19.  Can  rays  of  light  cross  one  another  without  interfering  with 
the  effect  of  the  rays  ? 

20.  The  two  sides  of  the  greased  paper  photometer  (§  438,6.)  are 
equally  illuminated  by  a  candle  flame  on  one  side,  distant  18  inches, 
and  by  a  lamp  flame  on  the  other  side,  distant  6  feet.     How  does 
the  illuminating   power  of  the  lamp   compare  with  that  of  the 
candle  ? 

21.  Are  all  the  luminous  rays  of  the  same  color? 

22.  Which  is  the  more  bulky,  a  pound  of  ice  or  a  pound  of  water  ? 

23.  How  can  you  make  water  boil  at  a  lower  temperature  than 
212°  F. ? 

24.  What  is  an  anode?    A  node? 

25.  Does  the  vaporization  of  water  change  the  size  of  the  mole- 
cules or  the  distances  between  them  ? 

26.  How  do  you  change  centigrade  into  Fahrenheit  readings  ? 

27.  What  piece  or  pieces  of  physical  apparatus  have  you  made 
since  you  began  studying  this  book  ? 

28.  What  is  meant  by  ineitia  ? 

29.  How  far  will  a  freely  falling  body  move  in  10  seconds  ? 

30.  Which  is  the  more  general  term,  fluid  or  liquid? 

31.  Give  a  general  law  of  machines. 

32.  How  can  energy  be  destroyed. 

33.  Define  force  and  energy. 

34.  State  Archimedes'  principle. 

35.  Define  specific  gravity. 

36.  Explain  gaseous  tension. 

37.  Describe  and  explain  the  experiment  with  the  Magdeburg 
hemispheres. 

38.  What  is  a  dynamo  ? 

39.  Explain  the  action  of  intermittent  springs. 

40.  Give  Ohm's  law. 


386 


NATURAL  PHILOSOPHY. 


§486 


41.  What  is  the  difference  between  noise  and  music  ? 

42.  Why  does  not  Hudson  Bay  freeze  solid  to  the  bottom? 

43.  What  is  an  ampere  ? 

44.  Explain  interference  of  sound. 

45.  What  is  meant  by  E.  M.  F.  ? 

46.  What  is  meant  by  the  "  mechanical  equivalent  of  heat "  ? 

47.  How  should  the  cells  of  a  voltaic  battery  be  joined  to  give 
the  best  effect  ? 

48.  On  what  optical  fact  does  the  success  of  a  sharpshooter 
(Fig.  340)  depend  ? 


FIG.  240. 
49.  Do  you  think  that  you  know  all  about  Natural  Philosophy* 


APPENDIX  A. 

Mathematical  Formulas. 
7r=3.14159. 
D=diameter. 
R— radius. 

Circumference  of  circle=7r  D. 
Area  of  a  circle=7r  R*. 
Surface  of  a  sphere=4  IT  R*=7r  D*. 
Volume  of  a  sphere  f  n  W=$  n  D». 

APPENDIX  B. 

International  or  Metric  Measures.— This  sys- 
tem bids  fair  to  come  into  general  use  in  this  country. 
For  this  reason,  as  well  as  for  its  greater  convenience,  an 
acquaintance  with  it  is  now  desirable  and  may  soon  be 
necessary.  It  has  been  already  legalized  by  act  of  Con- 
gress. The  meter  is  defined  as  the  forty-millionth 
of  the  earth's  meridian  which  passes  through  Pans. 
It  is  equal  to  39.37  inches.  Like  the  Arabic  system 
of  notation  and  the  table  of  U.  S.  Money,  its  divisions  and 
multiples  vary  in  a  tenfold  ratio. 


8  NATURAL  PHILOSOP&Y. 

Metric  Measures  of  Length.  —  Ratio  =  10. 

(Millimeter    (mm.)=          .001  m.=    0.03937  in. 
T"  \Centimeter   (cm.)  =         Loi    w.=    0.3937    " 


[Decimeter    (dm.)  =          .1      w.=     3.937 
UNIT.     Meter  (m.)  —        1.        m.=  39.37        " 

fDekameter   (Dm.)=      10.        m.  =393.7 
g  MTOTI-!  Hectometer  (JZ  TO.)  =     100.        m.  =328  ft.  1  in. 

1  PLES.  I  Kilometer    (Km.)  =  1000.        «i.=  0.62137  mi. 

lMyriameter(J/>».)=  10000.  m.=  65137  " 
3  -ZVote.  —  The  table  may  be  read  :  10  millimeters  make 
g  one  centimeter  ;  10  centimeters  make  one  decimeter,  etc. 
|  (Fig.  241.)  The  denominations  most  used  in  practice  are 
v  printed  in  italics.  The  system  of  nomenclature  is  very 
M  simple.  The  Latin  prefixes,  mitti-,  centi-  and  deci-,  sig- 

2  nifying  respectively  ^Vs*  -j-^,  and  -j^,  and  already  fa- 
«  miliar  in  the  mill,  cent  and  dime  of  U.  S.  Money,  are 
•s   used  for  the  divisions,  while  the  Greek  prefixes  deka-, 
"  hecko-,  kilo-  and  myria-,  signifying  respectively  10,  100, 
^   1000  and  10000,  are  used  for  the  multiples  of  the  unit. 
£   Each  name  is  accented  on  the  first  syllable.    It  may  be 
|  noticed  that  the  meter  corresponds  somewhat  closely  to 
S  the  yard,  which  it  will  replace.     Kilometers  will  be  used 
o   instead  of  miles. 

-  1  inch=  25.4000  TOOT.—  0.0254  m.  or  about  2|  cm. 
lfoot=  30.4800  cm.  =0.3048  m.  "  30  " 
1  yard=  0.9144m.  "  {?  of  a  meter. 

1  mile=1609.0000  m.    =1.6090  Km.     "      1T%  Km. 

Metric  Measures  of  Surface.—  Ratio  =io2 

FIG.  241.  =100. 

f  Square  millimeter  (sq.  7»»i.)=0.000001  sq.  m. 
DIVISIONS.  -|  Square  centimeter  («q.  cm.)  =0.0001          " 

[  Square  decimeter    (sq.  dm.)  =0.01 

UNIT.  Square  meter          (sq.  m.)    =1.  " 

etc.  ,  etc. 


APPENDIX.  389 

Note.—  The  table  may  be  read:  100  sq.  mm.=l  sq.  em. ;  100 
iq.  cm.  —  1  sq.  dm. ,  etc.  The  reason  for  the  change  of  ratio  from 
10  to  100  may  be  clearly  shown  by  representing  1  sq.  dm.,  and 
dividing  it  into  sq.  cm.  by  lines,  which  shall  divide  each  side  of 
the  sq.  dm.  into  10  equal  parts  or  centimeters. 

Metric  Measures  of  Volume.— Ratio =io3  = 
1000. 

f  Cubic  millimeter  (cu.  mm.)=0.000000001  cu.  m, 
DIVISIONS.  \  Cubic  centimeter  (cu.  cm.)  =0.000001         " 

[Cubic  decimeter  (cu.  dm.)  =0.001  " 

UNIT.  Cubic  meter          (cu.  m.)    =1.308  cu.  yds. 

etc.  etc. 

Metric    Measures    of   Capacity. — Ratio =10. — 

For  many  purposes,  such  as  the  measurement  of  articles 
usually  sold  by  dry  and  liquid  measures,  a  smaller  unit 
than  the  cubic  meter  is  desirable.  For  such  purposes 
the  cubic  decimeter  has  been  selected  as  the  standard, 
and  when  thus  used  is  called  a  liter  (pronouncecj 
leeter). 

fMilliliter(mZ.)    =       1   cu.cm.  =0.061022  cu.  in. 

DIVISIONS.    S  Centiliter  (cl.)   =     10        "  =0.338  fld.  oz. 

1  Deciliter  (<B.)     =100        "  =0.845  gill. 

UNIT.  Liter         (I.)    =1000        "  =1.0567  liquid  qt. 

f  Dekaliter  (Dl.)  =  10  CM,  dm.  =9.08  dry  qt. 

MULTIPLES.  ]  Hektoliter  (#£.)= 100  cu.  dm.  =2  bu.  3.35  pk. 

[Kiloliter    (El.)  =     1  cu.  m.  =264.17  gal. 

Fortunately,  there  is  but  one  liter.  This  is  intermediate,  in 
value,  between  the  dry  and  the  liquid  quarts  which  it  will  replace. 
The  dekaliter  does  not  differ  very  much  from  the  peck  and  might 
be  substituted  for  it  without  much  confusion. 


390  NATURAL  PfffLOSOPHT. 

1  U.  8.  liquid  quart  =  0.946  J.  or  about   1  liter. 

1  U.  S.  dry        "  =  1.101  1.  "           1     " 

1U.  S.  gallon  =  3.785 1.  "         8  r80  " 

1  U.  8.  bushel  =35.240 1.  "         T*T  of  a  hektoliter. 

Metric  Measures  of  Weight. — Ratio  =  10. 

("  Milligram  (mg.)  =  0.0154  grains  avoirdupois. 

DIVISIONS.    \  Centigram  (eg.)    =  0.1543        "              " 

{ Decigram  (dg,)  =  1.5432 

UNITS.             Gram  (g.)     =  15:483         " 

f  Dekagram  (Dg.)  =  0.3527  oz.                 " 

I  Hektogram  (Eg.)  —  3.5274    " 

MULTIPLES,  j  Kilogram  (Kg.)  =  2.2046  Ibs. 

(.Myriagram  (Mg.)  =  22.046      " 

A  gram  is  the  weight  of  one  cubic  centimeter 
of  pure  water,  at  its  temperature  of  greatest  density 
(4°  C.  or  39.2°  F.).  A  5-cent  nickel  coin  weighs  five 
grams.  Fortunately,  there  is  but  one  gram  or  one  kilo- 
gram. 

1  avoirdupois  ounce  =  28.35  g.  or  a  little  less  than  30  grams. 
1  Troy  or  apothecaries  ounce=31.10  g.  or  a  little  more  than  30  grams. 
1  avoirdupois  pound  453. 59  g.  or  about  TST  of  a  kilogram. 

The  best  way  for  the  pupil  to  become  familiar  with  these 
weights  and  measures  is  to  use  them.  He  will  quickly  discover 
that  very  many  computations  with  these  units  consist  only  of  in- 
telligent shiftings  of  the  decimal  point. 


APPENDIX.  391 


EXERCISES. 

1.  How  much  water,  by  weight,  will  a  liter  flask  contain  ? 

2.  If  sulphuric  acid  is  1.8  times  as  heavy  as  water,  what  weight 
of  the  acid  will  a  liter  flask  contain  ? 

3.  If  alcohol  is  0.8  times  as  heavy  as  water,  how  much  will  1360 
CM.  cm.  of  alcohol  weigh  ? 

4.  What  part  of  a  liter  of  water  is  250  g.  of  water? 

5.  What  is  the  weight  of  a  cu.  dm.  of  water? 

6.  What  is  the  weight  of  a  cU.  of  water? 


NUMBERS  REFER  TO  PARAGRAPHS,  UNLESS  OTHERWISE  INDICATED^ 


A. 

Absolute  temperature,  366. 

"        zero,  366. 
Absorption  of  heat,  403,  404. 
Acid  for  batteries,  248*. 
Acoustic  tubes,  331. 
Actinic  spectrum,  469. 
Adhesion,  29,  30. 
Aeriform  bodies,  40,  41,  180. 
Air,  180,  181. 
"    chamber,  193. 
"    pump,  187. 

Amalgamating  battery  zincs,  354. 
Ampere,  351. 

Amplitude  of  wave,  322,  330. 
Analysis  of  solar  light,  464. 
Angle,  Critical,  446. 

"      of  incidence,  57. 

"      of  reflection,  57. 
Anion,  272. 
Anode,  272. 

Aperture  of  mirror,  435. 
Apparent  direction  of  bodies,  433. 
Appendix,  p.  387. 
Aqueous  humor,  473. 
Archimedes'  principle,  162. 
Armatures,  281,  310. 
Artesian  well,  Ex.  14,  p.  139. 
Artificial  magnets,  205,  281. 
Astronomical  telescopes,  477,  478. 
Athermanous,  400. 
Atmospheric  electricity,  242. 
Atmospheric  pressure,  183,  185. 
Atom  defined,  3. 
Atomic  attraction,  7. 

-      motion,  8. 


Attraction,  Electric,  198,  335. 
"          Forms  of,  7. 
"          Magnetic,  284,  «86. 
Axis  of  lens,  451. 
44      mirror,  435. 

B. 

Balance,  113-114. 
Barometer,  183. 
Base,  72. 
Batteries,  Brush,  274^. 

44         Bunsen,  Fig.  no. 

44         Electric,  264-367. 

"         Faure,  2740. 

44         Grove,  Fig.  109. 

44         Secondary,  374. 

44         Storage,  274. 
Battery,  see  Cell  or  Element 

"  zincs,  254. 
Beam  of  light,  424. 
Beats,  345. 

Bellows,  Hydrostatic,  149. 
Blake  transmitter,  337. 
Blue  vitriol,  260. 
Boiling  point,  360,  372. 
Breast  wheel,  176. 
Brittleness,  29,  33. 
Broken  magnets,  287. 
Brush  battery,  274^;  Ex.  33,  p.  144 

41     lamp,  313. 
Bunsen  cell  or  battery,  363. 
Buoyancy,  162,  163. 
Burning  glass,  4520. 

c. 

Callaud  cell  or  battery,  361. 


394 


INDEX. 


Calorific  powers,  410. 
Camera  obscura,  426. 

"        Photographer's,  472. 
Candle,  Standard,  Ex.  10,  p.  335. 
Capstan,  121. 

Carbon  dioxide  snow,  383^. 
Carbonic  acid  snow,  383^. 
Cathetal  prism,  Ex.  9,  p.  364. 
Cathion,  272. 
Cathode,  272. 
Cause  of  sound,  319. 
Cells,  Galvanic  or  Voltaic,  456. 
Centigrade  thermometer,  360. 
Centimeter,  App.  B. 
Centre  of  buoyancy,  163^. 
"        curvature,  435,  451. 
•*        gravity,  65,  66. 
M        oscillation,  Ezp.  38. 
Centrifugal  force,  52. 
Changes  of  physical  condition,  42. 

•»       Physical  and  chemical,  n. 
Characteristic   properties   of  matter, 

15,  28,  29. 
Charging  by  contact,  221. 

"  induction,  224. 

Chemical  action  develops  electricity, 

246. 

Chemical  action  produces  heat,  410. 
"        changes,  n. 
"        effects  of  the  electric  cur- 
rent, 271,  274. 
Chemistry,  10. 
Chromatics,  461. 
Circuit,  Electric,  249. 
Clocks,  90. 

Clouds,  Electrified,  242. 
Coercive  force,  282. 
Cohesion,  29,  30. 
Coincident  waves,  340. 
Colors,  465,  467. 

Combustion  produces  heat,  410. 
Communicating  vessels,  147,  161. 
Commutator,  311. 
Compass,  204. 

Composition  of  solar  light,  464. 
Compound  lever,  116. 
"          machines,  140. 
«*          masses,  5. 
«•          molecules,  33, 4*. 


Compound  tones,  353, 
Compressibility,  17,  25. 
Concave  lens,  450,  458,  459. 

"       mirror,  435-439. 

Concavo-convex  lens,  450. 

Condensation  of  gases,  383. 

Condenser,  Electric,  235,  236,  237. 

"  for  gas,  188. 

lens,  479. 

"  of  still,  374. 

Conditions  of  matter,  37. 
Conduction  of  electricity,  216,  220, 250. 

heat,  392. 

Conductive  discharge,  238,  241. 
Conductivity,  Electric,  216,  220,  250. 
"  Thermal,  of  gases,  394. 

liquids,  394. 
solids,  392- 
Conductors  of  electricity,  216,  220, 250. 

"  heat,  393. 

Conjugate  foci,  438,  453. 
Conservation  of  energy,  484. 
Constant  force,  77. 
Constitution  of  matter,  9. 
Continuity  of  matter,  6. 
Continuous  sounds,  327. 
Convection  of  heat,  395. 
Convective  discharge,  238,  240. 
Convertibility  of  energy,  100,  268. 
Convex-concave  lens,  450. 
Convex  lenses,  450-457. 

"       mirrors,  440. 
Copper  plating,  Exp.  112. 
Copper  sulphate  cell  or  battery,  261. 
Cornea,  473. 

Correlation  of  energy,  485. 
Coulomb,  253. 
Critical  angle,  446. 
Crystalline  lens,  473. 
Current  electricity,  200,  201,  206,  247, 

3°3- 

Curvature,  Centre  of,  435,  451. 
Curves,  Magnetic,  290. 


Daniell's  cell  or  battery,  260. 
Day  dream,  355. 
Decimeter,  App.  B. 


TNDEX. 


395 


Declination,  Magnetic,  297. 

Electric  density,  231. 

Dekameter,  App.  B. 

"       discharges,  238-241. 

Density,  Electric,  231. 

"       energy,  209. 

Diamagnetic  substances,  288. 

"       experiments,  pp.  174-176. 

Diathermancy,  400. 

"      fluids,  244- 

Diffusion  ot  heat,  392-404. 

"       force,  209. 

light,  432. 

"       induction,  222,  304. 

Dip,  Magnetic,  296. 

"        insulators,  216. 

Dipper,  The  Great,  Exp.  136. 

"       lamps,  312,  313. 

Dipping  needle,  295  a.  and  d. 

"       light,  312,  313. 

Direction,  Line  of,  71. 

machines,  232-234,  310,  311. 

"         of  bodies,  Apparent,  433. 

"       manifestations,  210. 

Discharger,  Electric,  237. 

"       motor,  31  1£. 

Discharges,  Electric,  238-241. 

"       pendulum,  199. 

Dispersion  of  light,  463. 

"       polarization,  222. 

Disruptive  discharge,  238,  239. 

"       pojes,  249. 

Distillation,  374. 

"       potential,  218. 

Distribution  of  Electricity,  230,  231. 

"       repulsion,  199. 

Diverging  meniscus,  450. 

"       resistance,  220,  268. 

Divisibility,  17,  23. 

"       spark,  242. 

Divisions  of  matter,  2. 

"       telegraph,  276,  298. 

Double  concave  lens,  450. 

"       tension,  217. 

"       convex  lens,  450. 

"       whirl,  Exp.  103. 

"       weighing,  114. 

Electricity,  196-315. 

Downward  liquid  pressure,  152. 

"          Atmospheric,  243. 

Dropping  bottle,  Fig.  192. 

"           Distribution  of,  230,  231. 

Ductility,  29,  35. 

"           Dynamic,  247. 

Duration  of  electric  spark,  242. 

Frictional,  199,  209-244. 

Dynamo-electric  machines,  310,  311. 

"          Galvanic,  201,  246-273. 

Dynamos,  311. 

"          Induced,  303-315. 

Dynamics,  48. 

"          Relation  of,  to  energy,  229, 

244. 

E. 

Static,  199,  209-244. 

"           Theory  of,  226. 

Earl  of  Rosse,  478. 

"           Thermo-,  206,  247,  278. 

Ebullition,  37t,  372. 

"           Voltaic,  201,  246-273. 

Edison,  Ex.  22,  p.  244. 

Electrodes  249. 

Effect  of  concave  mirrors,  436. 

Electrolysis,  271. 

Elasticity,  17,  27,  55. 

Electrolytes,  271. 

Electric  attraction,  198,  225. 

Electro-magnets,  275^,  298. 

bells,  Exp.  95,  Fig.  105. 

Electro-motive  force,  219. 

"       charges,  209-244. 

Electrophorus,  227-229. 

circuit,  249. 

Electro  plating,  Exit.  112. 

"       condenser,  233-237. 

Electroscopes,  215,  Ex.  3,  p.  179. 

"       conduction,  221. 

Electrostatics,  Law  01,  214. 

"       conductors,  216. 

Elementary  masses,  5. 

"       current,  200,  201,  206,  303. 

"            molecules,  30.,  +t. 

"             "        Effects  of,  268-271. 

Elements,  5. 

"        Extra,  305. 

E.  M.  F.,  219. 

INDEX. 


Energy,  U.,  93-103,  144.  229-244,  301, 

Fundamental  tone,  347,  348. 

407,  418,  483-485- 

Fusion,  Latent  heat  of,  378. 

Engines,  Steam,  414-418. 

"       Laws  of,  368. 

Equilibrium,  67-70,  160,  161. 

Escapement  of  clocks,  90. 

G. 

Ether,  Luminiferous,  396. 
Evaporation,  369,  370. 
Exchange,  Telephone,  3373. 

Galilean  telescope,  476. 
Galvanic  battery,  264-267. 
"         cell  248  256. 

Expansibility,  17,  26. 

Expansion,  8c,  361-365. 
Extension,  17,  18. 
Extra  current,  305. 

"         electricity,  201,  246-273. 
"        element,  248,  256. 
Galvanometer,  277. 

Eye,  473. 
"    glass,  475. 

Galvanoscope,  277. 
Gases,  41. 

F. 

"      Condensation  of,  383. 

Fahrenheit's  thermometer,  360. 
Falling  bodies,  75-82. 

"      Expansion  of,  365. 
"      Thermal  conductivity  of,  394. 

False  balance,  113. 

44      Type  of,  1  80. 

Farsightedness,  473*. 
Field,  Magnetic,  290. 
Floating  bodies,  163. 
Flow  of  liquids,  171. 

"      Volume  of,  Exp.  7,  p.  137. 
Geissler  tubes,  ooo. 
Graduation  of  thermometers,  360. 
Gram,  App.  B. 

"        rivers,  172. 
Fluids,  44- 
Thermal  conductivity  of,  394. 
Fly  wheel,  415^. 
Focal  distance,  435,  452. 
Foci,  Conjugate,  438,  4524?.,  453. 
"     Secondary,  t,ytd. 
Focus  defined,  437. 
"     of  lens,  452-459. 
"     of  mirror,  435-440. 

Gravitation,  59. 
"           Laws  of,  60. 
Gravity,  20,  61,  77. 
"       cell  or  battery,  261. 
"        Centre  of,  65. 
"       Increment  of,  81. 
"       Specific,  165-169. 
Great  Dipper,  Exp.  136. 
Grove  cell  or  battery,  263. 

Foot-pound,  95. 

u 

Force,  47. 

n. 

"     Centrifugal,  52. 

Hand  glass,  Exp.  6r. 

"     Constant,  77. 

Hardness,  29,  31. 

"     pump,  191-193- 

Harmonics,  347. 

Forms  of  attraction,  7. 

Head  of  liquids,  171. 

"        motion,  8. 

Heat,  Absorption  of,  403,  404. 

Formulas,  Mathematical,  App.  A. 

"     Conduction  of,  392. 

Fountain  of  fire,  450^. 

"     Convection  of,  395. 

"         in  vacuo,  Exp.  65. 

"     defined,  357. 

Freely  falling  bodies,  78. 

"     Diffusion  of,  392-404. 

Freezing  mixtures,  380. 

"     from  chemical  action,  410. 

Friction,  142-144. 

"         "    combustion,  410. 

"       produces  heat,  409. 

"         "    electric  current,  268. 

Frictional  electricity,  199,  209-244. 

"         "    friction,  409. 

Fulcrum,  109. 

"        "    mechanical  energy,  354. 

INDEX. 


397 


Heat  from  percussion,  408. 

Insulators,  216. 

"     Latent,  377-386. 

Intensity  of  light,  428. 

"     Luminous,  401. 

"              sound,  330. 

"     Obscure,  401. 

Interference  of  sound,  344. 

"     Radiant,  397,  403,  420. 

Intermolecular  spaces,  6,  8r. 

"     Radiation  of,  398,  404. 

Internal  reflection  of  light,  445. 

"     Reflection  of,  402,  404. 

"        resistance,  Electric,  250. 

"     Refraction  of,  402. 

International  measures,  App.  B. 

"     related  to  energy,  407. 

Inverted  image-  ,  426. 

"     Sensible,  377,  4°3- 

Invisible  spectrum,  468. 

"     Specific,  387-390. 

Ions,  272. 

"     unit,  376. 

Ins,  473. 

Heating  powers,  411. 

Hectometer,  App.  B. 

J. 

Heliostat,  p.  336. 

Helmholtz's  resonator,  343. 

Joule's  equivalent,  413. 

Hiero's  fountain,  Ex.  4,  p.  137. 

Holtz  electric  machine,  note,  p.  168. 

K. 

Homogeneous  medium,  425. 

Horse  power,  97. 

Kathion,  272. 

Horizontal  needle,  2953. 
Hydrogen,  Specific  heat  of,  390. 
Hydrokinetics,  171-177. 
Hydrostatic  bellows,  149. 
"         press,  150. 

Kathode,  272. 
Kilogram,  App,  B. 
Kilogrammeter,  95*. 
Kilometer,  App.  B. 
Kinetic  energy,  99. 

1 

"       theory  of  gases,  179. 

Images  formed  by  lenses,  454-459. 

L. 

"          mirrors,  439,  440. 

"      Inverted,  426. 

Latent  heat,  377-386. 

"      Projection  of,  439. 

Lateral  liquid  pressure,  157. 

"      Real,  439. 

"       inversion,  Exp.  216. 

"      Virtual,  434, 

Lamps,  Arc,  313. 

Impenetrability,  17,  19, 

"       Incandescence,  312. 

Incidence,  Angle  of,  57. 

Law  of  boiling,  372. 

Inclination,  Magnetic,  296. 

"      ebullition,  372. 

Inclined  plane,  133,  134. 

"      electrostatics,  214. 

Increment,  of  gravity,  81. 

"      falling  bodies,  79,  82. 

"            velocity,  81. 

"      fusion,  368. 

Indestructibility  of  energy,  103. 

gravitation,  60. 

"                  "   matter,  17,  21. 

inclined  plane,  134. 

Induced  currents,  303-315. 

inertia,  51. 

"       electricity,  206,  247,  303. 

lever,  in,  116. 

Induction  coil,  306. 

luminous  intensity,  428. 

"         Electric,  32a,  304. 

machines,  108. 

Magnetic,  390. 

magnets,  286. 

Inertia,  17-22. 

melting,  368. 

"        Laws  of,  51. 

motion,  50-54. 

Initial  velocity  of  falling  bodies,  80. 

pendulum,  86-88. 

INDEX. 


Law  of  pulley,  131. 

Luminous  pencil,  424. 

'      reflected  motion,  57. 

ray,  423. 

"      reflection  of  light,  431. 

"         spectrum,  467. 

"      refraction  of  light,  444. 

Luminiferous  ether,  396. 

"      screw,  139. 

"      thermodynamics,  413. 

M. 

"      weight,  63. 

"      wheel  and  axle,  120. 

Machines,  105,  140. 

"       Ohm'  s,  252. 

Machines  cannot  create  energy,  106. 

Leaning  towers,  733. 

"         Compound  140. 

Leclanche  cell  or  battery,  259. 

defined,  105. 

Lens,  447,  450-459,  473- 

"         Electric,  232-234,  310,  311. 

Lever,  109-116. 

"         Laws  of,  108. 

"      Classes  of,  no. 

"         Simple,  105-139. 

"      Compound,  116. 

"         Uses  of,  107. 

"      Laws  of,  in. 

Magic  lantern,  479. 

Leyden  jar,  235-237. 

Magdeburg  hemispheres,  Exp.  66. 

Lifting  pump,  189,  190. 

Magnetic  attraction,  284,  286,  293. 

Light,  Analysis  of,  464. 

"        curves,  290. 

"      defined,  420. 

"         declination,  297. 

"      Diffused,  432. 

dip,  296. 

"      Dispersion  of,  463. 

"         effects   of    electric    current 

"      Electric,  312,  313. 

275-277. 

"      Intensity  of,  428. 

"         equator,  283  ;  field,  290. 

"      Rectilinear  motion  of,  425. 

"        force,  Lines  of,  290. 

"      Reflection  of,  430-440. 

"         inclination,  296. 

"      Refraction  of,  442-459. 

"         induction,  292. 

•'      Synthesis  of,  464. 

•*        needles,  295. 

"      Total  reflection  of,  445. 

"         neutral  point,  283. 

"      Velocity  of,  427. 

"        poles,  283. 

Lightning,  242. 

*'         screens,  289. 

"          rods,  243. 

"        substances,  288. 

Line  of  direction,  71. 

"         variation,  297. 

Liquid,  39. 

Magnetism,  196-315. 

"       pressure,  146-158. 

"           related  to  energy,  301. 

Liquids,  Equilibrium  of,  160. 

"            Residual,  299*. 

"        in    communicating    vessels, 

Magnetization,  291,  300. 

161. 

Magnetized  substances,  288. 

"       Thermal  conductivity  of,  394. 

Magneto-electric  currents,  308-315. 

Liter,  App.  B. 

"              "      machines,  311*. 

Loadstone,  205,  280. 

Magnets,  102-206,  280-300. 

Local  action,  254. 

"         Artificial,  281. 

Locomotive,  416. 

"        Broken,  287. 

Lodestone,  205,  280. 

Electro-,  275*.,  298. 

Loudness  of  sound,  330. 

"         How  made,  291,  300. 

Luminous  beam,  424. 

"         Laws  of,  286. 

body,  42.. 

"         Making,  291,  300. 

"         effect  of  electricity,  242,  269. 

"         Molecular,  287. 

14         heat,  401. 

"        Natural,  280. 

399 


Magnets,  Neutral  point  of,  383. 

Motions  of  molecules,  8. 

"        Planetary,  294. 

Motor,  Electric,  311*. 

"         Sucking,  ajy. 

Music,  328,  329. 

Magnifying  glasses,  474. 

Malleability,  29,  34. 

N. 

Mariner's  compass,  295*1. 

Mass  denned,  5. 

Natural  magnets,  205,  280. 

"     attraction,  7. 

"       philosophy,  defined,  13. 

"     motion,  8. 

Nature  of  electricity,  209. 

Mathematical  formulas,  App.  A. 

Nearsigatedness,  473*1. 

Matter  defined,  i. 

Needle,  Magnetic,  295. 

"       Conditions  of,  37. 

Neutral  equilibrium,  70. 

"       Constitution  of,  9. 

"       point  of  magnet,  283. 

"      Continuity  of,  6. 

Newton's  disc,  Exp.  233. 

"       Divisions  of,  2. 

"        laws  of  motion,  50-54. 

"       Properties  oif,  12. 

Nodal  points  ;  nodes,  347. 

"       Radiant,  43. 

Noise,  328. 

"      Ultra  gaseous  form  of,  43. 

North  star,  Exp.  136. 

"       Measure  of  work,  94-97. 

Measures,    Metric    or     international, 
App.  B. 

0. 

Mechanical  effects  of  electricity,  198. 

Ohm  defined,  220. 

"          energy  from  heat,  356. 

Ohm's  law,  252. 

'•          equivalent  of  heat,  413. 

Object  glass  ;  objective,  475. 

"          motion,  8. 

Obscure  heat,  401. 

Mechanics,  48*1. 

Opaque  bodies,  422. 

Megohm,  220. 

Opera  glass,  476. 

Meniscus,  450. 

Optical  centre,  451. 

Meter,  metric  measures,  App.  B. 

"       instruments,  472-481. 

Microscope,  Simple,  474. 

Optic  nerve,  473. 

Compound,  475. 

Optics,  420-481. 

Millimeter,  App.  B. 

Oscilliation,  Centre  of,  Exp.  38. 

Mirror,  Concave,  435. 

"          of  pendulum,  85. 

"       Convex,  440. 

Overshot  wheel,  175. 

"       Plane,  434. 

Overtones,  347-351- 

Molecular  attraction,  7. 

"          magnets,  287. 
u          motion.  8. 

P. 

Molecule  defined,  4. 

Pascal's  experiment,  148,  184., 

Molecules,  Motions  of,  8. 

"       principle,  148. 

Momentum,  49. 

Pendulum,  84-90. 

Motion  defined,  46. 

Pencil  of  light,  424. 

"       Forms  of,  8. 

Percussion  produces  heat,  408. 

"       Newton's  laws  of,  50-54. 

Permanent  magnets,  203. 

"       of  pendulum,  85. 

Philosophy,  Natural,  defined,  13. 

"       Reflected,  56. 

Photographer's  camera,  472. 

Law  of,  57. 

Photometer,  4280.  and  b. 

Motions  of  atoms,  8. 

Photometry,  4280. 

"         masses,  8. 

Physical  changes,  n. 

400 


INDEX. 


Physical  properties,  15. 

"        science,  10. 
Physics,  deflned,  13, 
Physiological  effects  of  electric  cur- 
rent, 270. 
Pinion,  122 
Pipette,  Exp.  54. 
Pitch  ot  sound,  332. 
Plane,  Inclined,  133,  134. 
Plano-concave  lens,  450. 

"     convex  lens,  450. 
Plate  electric  machine,  233,  234. 
Plates,  Refracting,  447,  448. 
Plating,  Electro,  Exp.  112. 
Pneumatics,  178. 

Pointed  conductors  of  electricity,  231. 
Polarization,  Electric,  222,  225,  255. 

"  Electromotive    force  of, 

273. 
Poles,  Electric,  249. 

"      Magnetic,  283. 
Porosity,  17,  24. 
Porte  lumiere,  p.  336. 
Position,  Energy  of,  99. 
Potassium  dichromate  cell  or  battery, 

258. 
Potential,  Electric,  218. 

"         energy,  99. 
Press,  Hydrostatic,  tyt. 
Pressure,  Atmospheric,  182,  185. 

"         of  liquids,  Downward,  15*. 

"  "        Sidewise,  157. 

"        Upward,  155. 

"         transmitted  by  liquids,  146. 
Primary  coil,  304. 
Prince  Rupert  drop,  Exp.  44. 
Principal  axis  of  lens,  451. 

"  '         mitror,  435. 

"        focus.  435,  437,  452. 
Prism,  447,  449,  450*1. 

"       Cathetal,  Ex.  o,  p.  364. 
Projection  of  images,  439. 
Propagation  of  sound,  320,  331. 
Properties  of  matter,  12,  15. 
Pulley,  126-132. 
Pumps,  187-193. 
PupU  of  eye,  473. 


Q. 

Quality  of  sound,  329,  351. 


Radiant  heat,  397,  403,  420. 

"       matter,  43. 
Radiation  of  heat,  398,  404. 
Ray  of  heat,  399. 

"      light,  423. 
Rainbow,  466. 
Reaction,  54,  55. 
Real  images,  439,  455,  456. 
Rectilinear  motion  of  light,  425. 
Reflected  motion,  56,  57. 
Reflector  telescope,  478. 
Reflection,  Angle  of,  57. 

of  heat,  402,  404, 
light,  430-440. 
sound,  333. 

"  Total,  internal,  445. 

Refraction  explained,  443. 
"•       of  heat,  402. 
light,  442. 
Refractors,  447. 
Refractor  telescope,  477. 
Reinforcement  of  sound,  341. 
Repulsion,  Electric,  199. 
Residual  magnetism,  299*1. 
Resinous  electricity,  213. 
Resistance,  Electric,  220,  250,  268. 
Resonance-,  342. 
Resonators,  Helmholtz's,  343. 
Retentivity  of  magnets,  282. 
Retina,  473. 
Retort  of  still,  374. 
RuhmkorfTs  coil,  306. 

s. 

Savart's  wheel,  332«. 
Sciopticon,  479. 
Screw,  138,  139. 
Secondary  axis  of  lens,  451. 

"  "         mirror,  435. 

"         battery,  274. 

"         coil,  304. 
foci,  452^. 

Second's  pendulum,  89. 
Segments  of  strings,  347. 
Sensible  heat,  377,  403. 


INDEX. 


401 


Simple  machines,  105-139. 

Stereoscope,  481. 

"      tones,  352. 

Stereoscopic  effects,  480. 

Siphon,  194. 

Storage  battery,  274. 

Slide,  Magic  lantern,  479. 

Straight  line  motion  of  light,  425. 

Sliding  valve,  415. 

Strings,  Vibrations  of,  346. 

Smee's  cell  or  battery,  257. 

Sulphate  of  copper  cell  or  battery,  261. 

Solar  spectrum,  462. 

Surveyor's  compass,  295*1. 

Solidification,  381. 

Swan  lamp,  312,  Ex.  23,  p.  244. 

Solids,  38. 

Sympathetic  vibrations,  338,  342. 

Solids,  Expansion  of,  362. 

Synthesis  of  white  light,  464. 

"      Thermal  conductivity  of,  393. 

Syphon,  194. 

Solution,  Latent  heat  of,  379. 

So  ometer,  23801. 

T. 

So  nd  beats,  345. 

Cause  of,  319. 

Telegraph,  Electric,  276. 

Continuous,  327. 

Telephone,  Electric,  335. 

defined,  317. 

"                "        Action  of,  336, 

Intensity  of,  330. 

"          exchange,  3373. 

Interference  of,  344. 

"          String,  Exp.  148. 

Loudness  of,  330. 

Telephonic  circuit,  315. 

media,  324. 

"          current,  314. 

Musical,  328,  329. 

"           transmitter,  337. 

Propagation  of,  320,  331. 

Telescopes,  476-478. 

Quality  of,  329,  351. 

Temperature,  358,  366. 

Reflection  of,  333. 

Temporary  magnets,  203. 

Reinforcement  of,  341. 

Tenacity,  29,  32. 

Transmission  of,  324. 

Tension,  Electric,  217. 

Velocity  of,  325,  3*6. 

"       of  gases,  179. 

waves,  318. 

Terrestrial  magnetism,  294. 

So  nding  boards,  339. 

Theory  of  electricity,  226. 

Spark,  Electric,  242. 

Thermal  effects  of  the  electric  current, 

Speaking  tubes,  331. 

268. 

Specific  gravity,  165-169. 

Thermal  spectrum,  470, 

"       heat,  387-39°. 

"       uuits,  376. 

Spectrum,  Solar,  462. 

Thermodynamics,  406-418. 

"         Actinic,  469. 

Thermo-electricity,  206,  247,  278. 

Invisible,  468-. 

"       electric  pile,  278. 

"         Luminous,  467. 

Thermometers,  359. 

"         Thermal,  470. 

Thermometric  scales,  360. 

Spherical  mirror,  435,  440. 

Timbre,  329,  351. 

Stable  equilibrium,  68. 

Time  pieces,  90. 

Stability,  73. 

Toepler-Holtz  electric  machine,  note. 

Standard  candle,  Ex.  10,  p.  335. 

p.  168. 

Static  electricity,  199,  209-244. 

Tones  and  overtones,  347. 

"     law,  note,  p.  72. 

"     Simple  and  compound,  352. 

Steam,  373. 

Torricelli's  experiment,  183. 

"      engine,  414-418. 

Total  reflection  of  light,  445. 

"      Latent  heat  of,  385. 

Towers,  Leaning,  73/1. 

Stereopticon,  479. 

Translucent  bodies,  423. 

402 


INDEX. 


Transmission  of  pressure  by  liquids, 

146. 

Transmission  of  sound,  324. 
Transmitter  of  telephone,  337. 
Transparent  bodies,  422. 
Tubes,  Acoustic,  331. 
Tuning  forks,  Mounted,  339*. 
Turbine  wheel,  174. 
Types  of  energy,  99. 

u. 

Ultra-gaseous  form  of  matter,  43. 
"     red  rays,  470. 
"     violet  rays,  469. 
Undershot  wheels,  177. 
Undulations,  318,  420*1. 
Undulatory  theory  of  light  and  heat, 

397*. 

Unit  of  heat,  376. 
"      work,  ys,  97, 
"      Thermal,  376. 

Universal  properties  of  matter,  13.  16. 
Unstable  equilibrium,  69. 
Upward  liquid  pressure,  155. 
Uses  of  machines,  107. 

V. 

Vaporization,  369. 

"  Latent  heat  of,  382. 

Vapors,  41. 

Variation,  Magnetic,  297. 
Varieties  of  energy,  483. 
Velocity,  Increment  of,  81. 

"         of  falling  bodies,  75,  80. 
of  light,  427. 
of  sound,  325.  326. 
"        related  to  energy,  98. 
Ventral  segments,  347. 
Vertical  needle,  29501. 
Vibrations  of  pendulum,  85-89. 
"        :of  strings,  346. 
"         Sympathetic,  338,  342. 
Virtual  images,  434,  457,  459. 


Vision,  Persistence  of,  Exp.  235= 
Vitreous  electricity,  213. 

"        humor,  473. 
Volt,  219. 
Voltaic  arc,  313. 
Voltaic  battery,  264-267. 

"       cell,  248,  256. 

"       current,  248. 

"       element,  248,  256. 

"       electricity,  201,  246-473. 
Voltameter,  271. 
Volume  of  gases,  Ex.  7,  p.  137. 

w. 

Water,  Expansion  of,  363,  364. 

"      jacket,  374^. 

Latent  heat  of,  384. 

"       lens,  4506. 

"      power,  173. 

"      Specific  heat  of,  390. 

"       voltameter,  271. 

"      wheels,  174-177. 
Wave,  Amplitude  of,  322. 

."      length,  321. 

"     motion,  318. 
Waves,  Coincident,  340. 
Wedje,  135-137: 
Weighing,  113,  ,14. 
Weight,  17,  20,  62. 
«'        of  air,  181. 

"       Law  of,  63. 
Wheel  and  axle,  118. 

"      work,  122-125. 
Whispering  galleries,  333*. 
White  of  the  eye,  473. 

"     vitriol,  261. 
Windlass,  121. 
Work,  92,  94-96- 
Worm  of  still,  374. 

z. 

Zero  of  temperature,  366. 

Zincs,  Amalgamating  battery,  354. 


This  book  is  DUE  on  the  last  date  stamped  below 


JWR  7      1934 


OCT 

c  '  • 

.OCT  1 
JiPB    W* 


Form  L-9-15m-7,'32 


A    000  941  341     0 


